WO2025253859A1 - Radiation-sensitive composition, pattern formation method, and onium salt compound - Google Patents

Abstract

Provided are: a radiation-sensitive composition having excellent sensitivity, CDU, MEEF, development defect suppressing properties, pattern circularity, and pattern rectangularity when forming a pattern; a pattern formation method; and an onium salt compound. This radiation-sensitive composition comprises: an onium salt compound represented by formula (1); a polymer that contains a structural unit having an acid-dissociable group; and a solvent. (In the formula, R¹ is a C4-C40 monovalent organic group. A^- is –SO₃ ^-, -COO^-, or –N^--SO₂-R^X. E¹ is -O-, -S-, -SO-, or -SO₂-. R² and R³ are each a hydrogen atom or a C1-C20 monovalent organic group. E is –O- or –NR^Y-. R⁴ and R⁵ are each a C1-C20 monovalent organic group, or R⁴ and R⁵ are bonded to represent a C4-C12 ring structure together with a sulfur atom. m is 0 or 1. n is an integer of 0-4.)

Description

Radiation-sensitive composition, pattern forming method, and onium salt compound

The present invention relates to a radiation-sensitive composition, a pattern formation method, and an onium salt compound.

Photolithography technology, which uses resist compositions, is used to form fine circuits on semiconductor elements. A typical procedure involves exposing a resist composition coating to radiation through a mask pattern, generating an acid, which then causes a reaction catalyzed by the acid, creating a difference in the solubility of the polymer in alkaline or organic developers between the exposed and unexposed areas, thereby forming a resist pattern on the substrate.

The above-mentioned photolithography technology is promoting the miniaturization of patterns by using short-wavelength radiation such as ArF excimer lasers, and also by using liquid immersion lithography, in which exposure is carried out while the space between the lens of the exposure device and the resist film is filled with a liquid medium. Lithography using even shorter-wavelength radiation such as electron beams, X-rays, and EUV (extreme ultraviolet) is also being considered as a next-generation technology.

Various structures have been investigated for photoacid generators, which are the photosensitive component that is the main component of resist compositions, including from the perspective of cationic structure (see JP 2022-68394 A).

JP 2022-68394 A

As patterns become increasingly finer, resist compositions are required to have various resist performance characteristics that are equal to or better than conventional ones in terms of sensitivity as well as critical dimension uniformity (CDU), which is an index of the uniformity of line width and hole diameter, mask error enhancement factor (MEEF), which is the amount of change in line width or hole diameter relative to the amount of change in mask size, suppression of development defects, pattern circularity, which indicates the circularity of the hole shape, and pattern rectangularity, which indicates the rectangularity of the cross-sectional shape of the resist pattern.

The present invention aims to provide a radiation-sensitive composition, pattern formation method, and onium salt compound that exhibit excellent sensitivity, CDU, MEEF, development defect suppression, pattern circularity, and pattern rectangularity during pattern formation.

As a result of extensive research into resolving this issue, the inventors discovered that the above objective could be achieved by adopting the following configuration, leading to the completion of the present invention.

That is, in one embodiment, the present invention provides:
An onium salt compound represented by the following formula (1) (hereinafter also referred to as "onium salt compound (1)"),
a polymer including a structural unit having an acid-dissociable group;
and a solvent.

(In formula (1),
^R1 is a monovalent organic group having 4 to 40 carbon atoms.
A ⁻ is —SO ₃ ⁻ , —COO ^— or —N ⁻ —SO ₂ —R ^X. R ^X is a monovalent organic group having 1 to 20 carbon atoms.
E ¹ is —O—, —S—, —SO— or —SO ₂ —.
^R2 is a hydrogen atom or a monovalent organic group having 1 to 20 carbon atoms.
E is —O— or —NR ^Y —, and ^{R Y} is a hydrogen atom or a monovalent hydrocarbon group having 1 to 10 carbon atoms.
^R3 is a hydrogen atom or a monovalent organic group having 1 to 20 carbon atoms.
^R4 and ^R5 are each independently a monovalent organic group having 1 to 20 carbon atoms, or ^R4 and ^R5 taken together represent a ring structure having 4 to 12 carbon atoms together with the sulfur atom to which they are bonded.
^R6 is a halogen atom, a hydroxy group, a nitro group, an amino group, a carboxy group, a cyano group, or a monovalent organic group having 1 to 20 carbon atoms. When a plurality of ^R6s are present, the plurality of ^R6s may be the same or different.
m is 0 or 1. When m is 1, both R ³ -E-CO- and R ² -E ¹ - are bonded to the 6-membered ring structure to which the sulfur atom in the above formula (1) is bonded.
n is an integer from 0 to 4.

Because this radiation-sensitive composition contains onium salt compound (1) as a radiation-sensitive acid generator or acid diffusion controller, it can exhibit excellent sensitivity, CDU, MEEF, development defect suppression, pattern circularity, and pattern rectangularity during pattern formation. Without being bound by any theory, the reason for this is presumed to be as follows: Because a substituent having an ester bond or amide bond is bonded to the aromatic ring of the sulfonium cation of onium salt compound (1), in addition to a substituent having an ether bond or a sulfur-containing bond, the polarity of onium salt compound (1) as a whole is enhanced. This allows for control of solubility in the developer, while also increasing compatibility with the base resin, thereby improving dispersibility in the resist film. Furthermore, because bonds that can extend the conjugated system, such as ether bonds, sulfur-containing bonds, ester bonds, and amide bonds, are bonded to the aromatic ring, the light absorption efficiency of the sulfonium cation is improved, thereby increasing the acid generation efficiency from onium salt compound (1). Furthermore, because the number of carbon atoms in the organic acid anion of onium salt compound (1) is within a specified range, the diffusion length of the generated acid can be appropriately controlled. It is believed that these combined effects enable the resist properties mentioned above to be achieved.

In another embodiment, the present invention provides
a step of directly or indirectly applying the radiation-sensitive composition to a substrate to form a resist film;
exposing the resist film to light;
and developing the exposed resist film with a developer.

This pattern formation method uses the above-mentioned radiation-sensitive composition, which has excellent sensitivity, CDU, MEEF, development defect suppression, pattern circularity, and pattern rectangularity, during pattern formation, making it possible to efficiently form high-quality resist patterns.

In yet another embodiment, the present invention provides
The present invention relates to an onium salt compound represented by the following formula (1a):

(In formula (1a),
R ^1a is a monovalent organic group having 5 to 40 carbon atoms.
A ⁻ is —SO ₃ ⁻ , —COO ^— or —N ⁻ —SO ₂ —R ^X. R ^X is a monovalent organic group having 1 to 20 carbon atoms.
E ¹ is —O—, —S—, —SO— or —SO ₂ —.
^R2 is a hydrogen atom or a monovalent organic group having 1 to 20 carbon atoms.
E is —O— or —NR ^Y —, and ^{R Y} is a hydrogen atom or a monovalent hydrocarbon group having 1 to 10 carbon atoms.
^R3 is a hydrogen atom or a monovalent organic group having 1 to 20 carbon atoms.
^R4 and ^R5 are each independently a monovalent organic group having 1 to 20 carbon atoms, or ^R4 and ^R5 are combined with each other to form a ring structure having 4 to 12 carbon atoms together with the sulfur atom to which they are bonded, provided that in the ring structure, the ring containing the sulfur atom in formula (1a) is fused with two other rings to form a tricyclic structure, and when the ring containing the sulfur atom in formula (1a) contains a heteroatom other than the sulfur atom, the heteroatom is an oxygen atom or a nitrogen atom.
^R6 is a halogen atom, a hydroxy group, a nitro group, an amino group, a carboxy group, a cyano group, or a monovalent organic group having 1 to 20 carbon atoms. When a plurality of ^R6s are present, the plurality of ^R6s may be the same or different.
m is 0 or 1. When m is 1, both R ³ -E-CO- and R ² -E ¹ - are bonded to the 6-membered ring structure to which the sulfur atom in the above formula (1a) is bonded.
n is an integer from 0 to 4.

Because the onium salt compound has the developer affinity, acid generation efficiency, and acid diffusion length described above, it is suitable as a radiation-sensitive acid generator or acid diffusion controller for radiation-sensitive compositions.

In this specification, the term "organic group" refers to a group containing at least one carbon atom (excluding groups that constitute functional or characteristic groups by themselves, such as a cyano group or a ketone group). A "fused ring structure" refers to a structure in which adjacent rings share one edge (two adjacent atoms). A "bridged ring hydrocarbon group" refers to a polycyclic cyclic hydrocarbon group in which two non-adjacent carbon atoms that constitute the ring are linked by a linking group containing one or more carbon atoms.

Embodiments of the present invention are described in detail below, but the present invention is not limited to these embodiments. Combinations of preferred aspects are also preferred.

<Radiation sensitive composition>
The radiation-sensitive composition according to this embodiment (hereinafter also referred to simply as the "composition") contains an onium salt compound (1), a polymer containing a structural unit having an acid-dissociable group (hereinafter also referred to as the "base polymer"), and a solvent. The composition may contain other optional components as long as the effects of the present invention are not impaired. By containing the onium salt compound (1) as a radiation-sensitive acid generator or an acid diffusion controller, the radiation-sensitive composition can exhibit excellent sensitivity, CDU, MEEF, development defect suppression, pattern circularity, and pattern rectangularity during pattern formation.

(Onium salt compound (1))
The onium salt compound (1) comprises an organic acid anion represented by the above formula (1) and a sulfonium cation, and functions as a radiation-sensitive acid generator or an acid diffusion controller. The function is determined by the organic acid anion. First, the sulfonium cation will be explained, followed by the organic acid anion.

In the above formula (1), examples of the monovalent organic group having 1 to 20 carbon atoms represented by ^R2 include a monovalent hydrocarbon group having 1 to 20 carbon atoms, a group (a) having a divalent heteroatom-containing group between carbon atoms of the hydrocarbon group (between two adjacent or non-adjacent carbon atoms) or at the end of the hydrocarbon group, a group in which some or all of the hydrogen atoms of the hydrocarbon group or the group (a) have been substituted with a monovalent heteroatom-containing group, and combinations thereof.

Examples of monovalent hydrocarbon groups having 1 to 20 carbon atoms include monovalent chain hydrocarbon groups having 1 to 20 carbon atoms, monovalent alicyclic hydrocarbon groups having 3 to 20 carbon atoms, monovalent aromatic hydrocarbon groups having 6 to 20 carbon atoms, and combinations of these.

Examples of monovalent chain hydrocarbon groups having 1 to 20 carbon atoms include alkyl groups such as methyl, ethyl, n-propyl, isopropyl, n-butyl, s-butyl, and t-butyl; alkenyl groups such as ethenyl, propenyl, and butenyl; and alkynyl groups such as ethynyl, propynyl, and butynyl.

Examples of monovalent alicyclic hydrocarbon groups having 3 to 20 carbon atoms include cycloalkyl groups such as cyclopentyl and cyclohexyl; cycloalkenyl groups such as cyclopropenyl, cyclopentenyl, and cyclohexenyl; bridged ring saturated hydrocarbon groups such as norbornyl, adamantyl, and tricyclodecyl; and bridged ring unsaturated hydrocarbon groups such as norbornenyl and tricyclodecenyl.

Examples of monovalent aromatic hydrocarbon groups having 6 to 20 carbon atoms include aryl groups such as phenyl, tolyl, xylyl, naphthyl, and anthryl, and aralkyl groups such as benzyl, phenethyl, naphthylmethyl, and anthrylmethyl.

Examples of heteroatoms constituting divalent or monovalent heteroatom-containing groups include oxygen atoms, nitrogen atoms, sulfur atoms, phosphorus atoms, silicon atoms, and halogen atoms. Examples of halogen atoms include fluorine atoms, chlorine atoms, bromine atoms, and iodine atoms.

Examples of divalent heteroatom-containing groups include -CO-, -CS-, -NR'-, -O-, -S-, -SO ₂ -, and combinations thereof, where R' is a hydrogen atom or a hydrocarbon group having 1 to 10 carbon atoms.

Examples of monovalent heteroatom-containing groups include hydroxy groups, sulfanyl groups, cyano groups, nitro groups, and halogen atoms. Examples of halogen atoms include fluorine atoms, chlorine atoms, bromine atoms, and iodine atoms.

^R2 is preferably a hydrogen atom, an alkyl group, a cycloalkyl group, an acyl group, an alkoxycarbonyl group, an alkoxycarbonylalkyl group, a cycloalkoxycarbonylalkyl group, an alkoxyalkyl group, a lactone structure-containing group (a group in which one hydrogen atom has been removed from a lactone structure), a group in which the hydrogen atom of such a group has been substituted with the above-mentioned monovalent heteroatom-containing group, or a combination thereof. Among these, ^R2 is preferably a hydrogen atom, a methyl group, an ethyl group, a carboxymethyl group, a t-butoxycarbonylmethyl group, a methylcyclopentyloxycarbonylmethyl group, an ethylcyclopentyloxycarbonylmethyl group, a methyladamantyloxycarbonylmethyl group, an ethyladamantyloxycarbonylmethyl group, an acetyl group, or a pivaloyl group.

^E1 is preferably —O— from the viewpoint of acid generation efficiency.

As the monovalent hydrocarbon group having 1 to 10 carbon atoms represented by R ^Y in E, a group corresponding to a carbon number of 1 to 10 among the monovalent hydrocarbon groups having 1 to 20 carbon atoms represented by R ² can be suitably used.

E is preferably -O- from the viewpoint of developer affinity.

As the monovalent organic group having 1 to 20 carbon atoms represented by ^R3 , a monovalent organic group having 1 to 20 carbon atoms represented by ^R2 can be suitably used.

^R3 is preferably a hydrogen atom, an alkyl group, a cycloalkyl group, an alkoxycarbonylalkyl group, a cycloalkoxycarbonylalkyl group, a lactone structure-containing group (a group in which one hydrogen atom has been removed from a lactone structure), a benzyl group, a hydroxyalkyl group, a group in which the hydrogen atom of these groups has been substituted with the above-mentioned monovalent heteroatom-containing group, or a combination thereof. Among these, ^R3 is more preferably a hydrogen atom, a methyl group, a 2-trifluoroethyl group, a t-butyl group, a t-amyl group, a methylcyclopentyl group, an ethylcyclopentyl group, a carboxymethyl group, a t-butoxycarbonylmethyl group, a methylcyclopentyloxycarbonylmethyl group, an ethylcyclopentyloxycarbonylmethyl group, a methyladamantyloxycarbonylmethyl group, or an ethyladamantyloxycarbonylmethyl group. It is also preferable that ^R3 contains an acid-dissociable group.

As the monovalent organic group having 1 to 20 carbon atoms represented by ^R4 and ^R5 , a monovalent organic group having 1 to 20 carbon atoms represented by ^R2 can be suitably used.

Examples of the ring structure having 4 to 12 carbon atoms formed by combining ^R4 and ^R5 together with the sulfur atom to which they are bonded include a sulfur atom-containing aliphatic heterocyclic structure having 4 to 12 carbon atoms and a sulfur atom-containing aromatic heterocyclic structure having 4 to 12 carbon atoms. Examples of the sulfur atom-containing aliphatic heterocyclic structure include thietane, tetrahydrothiophene, oxathiolane, thiane, dithiane, thiomorpholine, and thioxane. Examples of the sulfur atom-containing aromatic heterocyclic structure include thiophene, thiazole, benzothiophene, dibenzothiophene, and phenoxathiin. Among these, tetrahydrothiophene, thioxane, dibenzothiophene, benzothiophene, and phenoxathiin are more preferred as the ring structure formed by ^R4 and ^R5 .

The ring structure formed by ^R4 and ^R5 may have a substituent. Examples of the substituent include a halogen atom such as a fluorine atom, a chlorine atom, a bromine atom, or an iodine atom; a hydroxy group; a carboxy group; a cyano group; a nitro group; an alkyl group, an alkoxy group, an alkoxycarbonyl group, an alkoxycarbonyloxy group, an acyl group, an acyloxy group, or a group in which a hydrogen atom of any of these groups has been substituted with a halogen atom; and an oxo group (═O).

Preferably, ^R4 and ^R5 are each independently a substituted or unsubstituted phenyl group. The phenyl groups in ^R4 and ^R5 may have a substituent that the ring structure formed by ^R4 and ^R5 may have.

As the monovalent organic group having 1 to 20 carbon atoms represented by ^R6 , a monovalent organic group having 1 to 20 carbon atoms represented by ^R2 can be suitably used.

^R6 is preferably a hydroxy group, an alkyl group or a halogen atom, and more preferably a hydroxy group, a methyl group, a fluoro group, an iodo group, a trifluoromethyl group or a t-butyl group.

It is preferable that m is 0.

n is preferably an integer from 0 to 3, more preferably an integer from 0 to 2, and even more preferably 0 or 1.

In the above formula (1), it is preferable that R ³ -E-CO- is in an ortho position relative to R ² -E ¹ - (the carbon atom to which R ² -E ¹ - is bonded is adjacent to the carbon atom to which R ³ -E-CO- is bonded), which makes it possible to appropriately control the acid generation efficiency and to exhibit the above-mentioned resist performances at a higher level.

Specific examples of the sulfonium cation of onium salt compound (1) include, but are not limited to, structures represented by the following formulas (a-1) to (a-104):

As described above, onium salt compound (1) functions as either a radiation-sensitive acid generator or an acid diffusion controller depending on the structure of the organic acid anion. A radiation-sensitive acid generator is a compound that generates an acid that dissociates the acid-dissociable group upon exposure. An acid diffusion controller is a compound that generates an acid that does not dissociate the acid-dissociable group upon exposure, and functions to inhibit the diffusion of the acid generated from the radiation-sensitive acid generator in unexposed areas. The acid generated from the acid diffusion controller can be said to be a relatively weaker acid (having a higher pKa) than the acid generated from the radiation-sensitive acid generator. Whether onium salt compound (1) functions as a radiation-sensitive acid generator or an acid diffusion controller depends on factors such as the energy required to dissociate the acid-dissociable group in the base polymer and the acidity of the acid generated upon exposure. The radiation-sensitive composition may contain onium salt compound (1) in the form of a sole compound (free from the polymer), in the form of a polymer incorporated as part of the polymer, or both, although the form of a sole compound is preferred.

Acids that are generated upon exposure include those that produce sulfonic acids, sulfonimides, carboxylic acids, and sulfonamides. The organic acid anions can have structures corresponding to these.

Such acids include:
(1) A compound in which one or more fluorine atoms, fluorinated hydrocarbon groups, or cyano groups are substituted on the carbon atom at the α- or β-position of the sulfur atom of a sulfo group,
(2) a compound having a sulfonimide structure containing a fluorine atom,
(3) Compounds in which the carbon atom at the α-position or β-position to the sulfur atom of the sulfo group is not substituted with a fluorine atom, a fluorinated hydrocarbon group, or a cyano group.
(4) A compound in which one or more fluorine atoms or fluorinated hydrocarbon groups are substituted on the carbon atom adjacent to the carboxy group.
(5) A compound in which the carbon atom adjacent to the carboxy group is not substituted with a fluorine atom or a fluorinated hydrocarbon group.
(6) Compounds having a sulfonamide structure which may have a fluorine atom.

Among these, those corresponding to (1) and (2) above are preferred as radiation-sensitive acid generators. Those corresponding to (3) to (6) above are preferred as acid diffusion controllers, with those corresponding to (3) or (5) being particularly preferred. Even if the structure corresponds to (3) above, it can function as a radiation-sensitive acid generator if an electron-withdrawing group (such as a cyano group) is bonded to the carbon atom at the α- or β-position of the sulfur atom of the sulfo group.

When the onium salt compound (1) is a radiation-sensitive acid generator, the organic acid anion of the onium salt compound (1) is preferably represented by the following formula (za).

(In formula (za), R ^za is a monovalent organic group having 4 to 40 carbon atoms, provided that a fluorine atom, a monovalent fluorinated hydrocarbon group, or a cyano group is bonded to the carbon atom at the α- or β-position of —SO ₃ ^— .)

As the monovalent organic group having 4 to 40 carbon atoms represented by R ^za , a group in which the monovalent organic group having 1 to 20 carbon atoms represented by R ² in the above formula (1) is extended to have 4 to 40 carbon atoms can be suitably used.

R ^za is preferably a monovalent organic group having 4 to 40 carbon atoms and containing a cyclic structure. The organic group is not particularly limited and may be either a group containing only a cyclic structure or a group combining a cyclic structure with a chain structure. The cyclic structure may be a monocyclic ring, a polycyclic ring, or a combination thereof. The cyclic structure may be an alicyclic structure, an aromatic ring structure, or a combination thereof. In the case of a combination, the cyclic structures may be linked in a chain structure, or two or more cyclic structures may form a fused ring structure. These structures are preferably included as the smallest basic skeleton of the cyclic structure. The number of cyclic structures as the basic skeleton in the organic group may be one or two or more. The divalent heteroatom-containing group may be present between the carbon atoms forming the skeleton of the cyclic structure or the chain structure or at the terminal of the carbon chain, and hydrogen atoms on the carbon atoms of the cyclic structure or the chain structure may be substituted with other substituents.

As the alicyclic structure, a structure corresponding to the monovalent alicyclic hydrocarbon group having 3 to 20 carbon atoms in R ² of the above formula (1) can be suitably adopted.

As the aromatic ring structure, a structure corresponding to the monovalent aromatic hydrocarbon group having 6 to 20 carbon atoms in ^R2 of the above formula (1) can be preferably used. In addition, aromatic heterocycles such as a furan ring, a pyrrole ring, a thiophene ring, a phosphole ring, a pyrazole ring, an oxazole ring, an isoxazole ring, a thiazole ring, a pyridine ring, a pyrazine ring, a pyrimidine ring, a pyridazine ring, and a triazine ring are also suitable.

As the chain structure, a structure corresponding to the monovalent chain hydrocarbon group having 1 to 20 carbon atoms in R ² of the above formula (1) can be suitably adopted.

The alicyclic structure may also be an aliphatic heterocyclic structure.
Examples of the aliphatic heterocyclic structure include oxygen atom-containing aliphatic heterocyclic structures such as oxirane, tetrahydrofuran, tetrahydropyran, dioxolane, and dioxane;
Nitrogen atom-containing aliphatic heterocyclic structures such as aziridine, pyrrolidine, piperidine, and piperazine;
Sulfur atom-containing aliphatic heterocyclic structures such as thietane, thiolane, and thiane;
Examples include aliphatic heterocyclic structures containing multiple heteroatoms such as morpholine, 1,2-oxathiolane, and 1,3-oxathiolane.

Aliphatic heterocyclic structures include lactone structures, cyclic carbonate structures, sultone structures, cyclic acetal structures, cyclic imide structures, or combinations thereof.

R ^za preferably contains the above alicyclic structure.

As the substituent that substitutes the hydrogen atom on the carbon atom of the cyclic structure or chain structure, the substituent that the cyclic structure formed by ^R4 and ^R5 can have can be suitably used.

Examples of the monovalent fluorinated hydrocarbon group include monovalent fluorinated chain hydrocarbon groups having 1 to 20 carbon atoms, and monovalent fluorinated alicyclic hydrocarbon groups having 3 to 20 carbon atoms.

Examples of the monovalent fluorinated chain hydrocarbon group having 1 to 20 carbon atoms include fluorinated alkyl groups such as a trifluoromethyl group, a difluoromethyl group, a 2,2,2-trifluoroethyl group, a pentafluoroethyl group, a 2,2,3,3,3-pentafluoropropyl group, a 1,1,1,3,3,3-hexafluoropropyl group, a heptafluoro-n-propyl group, a heptafluoroisopropyl group, a nonafluoro-n-butyl group, a nonafluoroisobutyl group, a nonafluoro-t-butyl group, a 2,2,3,3,4,4,5,5-octafluoro-n-pentyl group, a tridecafluoro-n-hexyl group, and a 5,5,5-trifluoro-1,1-diethylpentyl group;
fluorinated alkenyl groups such as a trifluoroethenyl group and a pentafluoropropenyl group;
Examples include fluorinated alkynyl groups such as a fluoroethynyl group and a trifluoropropynyl group.

Examples of the monovalent fluorinated alicyclic hydrocarbon group having 3 to 20 carbon atoms include fluorinated cycloalkyl groups such as a fluorocyclopentyl group, a difluorocyclopentyl group, a nonafluorocyclopentyl group, a fluorocyclohexyl group, a difluorocyclohexyl group, an undecafluorocyclohexylmethyl group, a fluoronorbornyl group, a fluoroadamantyl group, a fluorobornyl group, a fluoroisobornyl group, and a fluorotricyclodecyl group;
Examples include fluorinated cycloalkenyl groups such as a fluorocyclopentenyl group and a nonafluorocyclohexenyl group.

The above-mentioned fluorinated hydrocarbon group is preferably a monovalent fluorinated chain hydrocarbon group having 1 to 8 carbon atoms, and more preferably a monovalent fluorinated straight-chain hydrocarbon group having 1 to 5 carbon atoms.

Specific examples of organic acid anions when onium salt compound (1) is a radiation-sensitive acid generator include, but are not limited to, structures represented by the following formulas (z-1-1) to (z-1-64) (including the structure represented by formula (z-a) above).

Onium salt compound (1) as a radiation-sensitive acid generator can be obtained by any combination of the above-mentioned sulfonium cation and the above-mentioned organic acid anion when onium salt compound (1) is used as a radiation-sensitive acid generator. Specific examples include, but are not limited to, structures represented by the following formulas (1B-1) to (1B-66).

The lower limit of the content of onium salt compound (1) as a radiation-sensitive acid generator (the total content when multiple types are included) is preferably 0.1 parts by mass, more preferably 1 part by mass, even more preferably 2 parts by mass, and particularly preferably 3 parts by mass, per 100 parts by mass of the base polymer described below. The upper limit of the content is preferably 100 parts by mass, more preferably 80 parts by mass, even more preferably 60 parts by mass, and particularly preferably 30 parts by mass. This enables the composition to exhibit excellent sensitivity, CDU, MEEF, development defect suppression, pattern circularity, and pattern rectangularity during pattern formation.

As long as the effects of the present invention are not impaired, the composition may contain a known radiation-sensitive acid generator other than the onium salt compound (1) as the radiation-sensitive acid generator.

When the onium salt compound (1) is an acid diffusion controller, the organic acid anion of the onium salt compound (1) is preferably represented by the following formula (z-b) or (z-c).

(In formulas (z-b) and (z-c), R ^zb and R ^zc each independently represent a monovalent organic group having 4 to 40 carbon atoms. However, in formula (z-c), no fluorine atom, monovalent fluorinated hydrocarbon group, or cyano group is bonded to the carbon atom at the α-position or β-position of —SO ₃ ^— .)

As the monovalent organic group having 4 to 40 carbon atoms represented by ^Rzb and ^Rzc , a monovalent organic group having 4 to 40 carbon atoms represented by ^Rza in the above formula (za) can be suitably used.

^Rzb and ^Rzc preferably contain at least one structure selected from the group consisting of a cyclic structure, a carbonyl group, and an ether bond. As the cyclic structure of ^Rzb and ^Rzc , the cyclic structure of ^Rza in the above formula (za) can be suitably adopted.

Specific examples of organic acid anions when onium salt compound (1) is an acid diffusion controller include, but are not limited to, structures represented by the following formulas (z-2-1) to (z-2-44) (including the structures represented by the above formulas (z-b) and (z-c)).

The onium salt compound (1) serving as an acid diffusion controller can be obtained by any combination of the above sulfonium cation and the above organic acid anion when the onium salt compound (1) is used as an acid diffusion controller. Specific examples include, but are not limited to, structures represented by the following formulas (1C-1) to (1C-34).

The lower limit of the content of onium salt compound (1) as an acid diffusion controller (the total content when multiple types are included) is preferably 0.1 parts by mass, more preferably 2 parts by mass, and even more preferably 4 parts by mass, per 100 parts by mass of the base polymer described below. The upper limit of the content is preferably 60 parts by mass, more preferably 50 parts by mass, and even more preferably 40 parts by mass. This enables the composition to exhibit excellent sensitivity, CDU, MEEF, development defect suppression, pattern circularity, and pattern rectangularity during pattern formation.

As long as the effects of the present invention are not impaired, the composition may contain a known acid diffusion controller other than the onium salt compound (1) as the acid diffusion controller.

(Method for synthesizing onium salt compound (1))
The onium salt compound (1) can be synthesized typically according to the following scheme. The following describes a case where, in the above formula (1), ^R4 and ^R5 are aryl groups, m and n are both 0, and ^R3 -E-CO- is bonded to the ortho position relative to ^R2 - ^E1- . However, the synthesis is not limited to this, and known methods can be used.

(In the scheme, R ¹ to R ³ , E ¹ , E, and A ⁻ are defined as in formula (1) above. Each Ar represents an aryl group. TfO ^{4 −} represents a trifluorosulfonate anion. M 4 ⁺ represents a monovalent alkali metal. ^{X −} represents a monovalent halide ion. Z ^{4 +} represents a monovalent cation.)

A diaryl sulfoxide and a benzoic acid derivative are reacted in the presence of a strong acid to form a sulfonium cation. This is then reacted with an alkali metal halide to form a halide salt, which is then reacted with a salt containing the desired organic acid anion to perform salt exchange, thereby synthesizing the desired onium salt compound (1). Because the first cationization step is a Friedel-Crafts type reaction, standard reactants used in such reactions can also be used. Other structures can also be synthesized by appropriately changing the starting materials, intermediate components, etc.

(polymer)
The polymer (i.e., base polymer) is an aggregate of polymer chains containing a structural unit having an acid-dissociable group (hereinafter also referred to as "structural unit (I)"). The "acid-dissociable group" refers to a group that substitutes a hydrogen atom in a carboxy group, a phenolic hydroxyl group, an alcoholic hydroxyl group, a sulfo group, or the like, and that dissociates under the action of an acid. The radiation-sensitive composition has excellent pattern formability because the polymer contains the structural unit (I).

In addition to structural unit (I), the base polymer preferably contains structural unit (II) containing at least one structure selected from the group consisting of a lactone structure, a cyclic carbonate structure, and a sultone structure, as described below, and may also contain structural units other than structural units (I) and (II). Each structural unit is described below.

[Structural unit (I)]
The structural unit (I) is a structural unit containing an acid-dissociable group. The structural unit (I) is not particularly limited as long as it has an acid-dissociable group, and examples thereof include a structural unit having a tertiary alkyl ester moiety, a structural unit having a structure in which the hydrogen atom of a phenolic hydroxyl group is substituted with a tertiary alkyl group, and a structural unit having an acetal bond. From the viewpoint of improving the pattern formability of the radiation-sensitive composition, a structural unit represented by the following formula (3) (hereinafter also referred to as "structural unit (I-1)") is preferred.

In the above formula (3), R ¹⁷ is a hydrogen atom, a fluorine atom, a methyl group, or a trifluoromethyl group. R ¹⁸ is a monovalent substituted or unsubstituted hydrocarbon group having 1 to 20 carbon atoms. ^{R 19} and R ²⁰ each independently represent a monovalent substituted or unsubstituted chain hydrocarbon group having 1 to 10 carbon atoms, a monovalent substituted or unsubstituted alicyclic hydrocarbon group having 3 to 20 carbon atoms, or a divalent alicyclic group having 3 to 20 carbon atoms formed by combining these groups together with the carbon atoms to which they are bonded. L ¹¹ represents ^* -COO-, ^* -L ^11a COO-, or ^* -COOL ^11a COO-. ^{L 11a} is a substituted or unsubstituted alkanediyl group or arenediyl group. * represents a bond to the carbon atom to which R ¹⁷ is bonded.

From the viewpoint of copolymerizability of the monomer that gives the structural unit (I-1), R ¹⁷ is preferably a hydrogen atom or a methyl group, more preferably a methyl group.

Examples of the alkanediyl group represented by L ^11a include alkanediyl groups having 1 to 10 carbon atoms, such as a methylene group, an ethanediyl group, a 1,3-propanediyl group, and a 2,2-propanediyl group. ^{L 11a} is preferably a methylene group or an ethanediyl group.

Examples of the arenediyl group represented by L ^11a include divalent aromatic hydrocarbon groups having 6 to 20 carbon atoms, such as benzenediyl and naphthalenediyl groups, with benzenediyl being preferred as ^{L 11a} .

Examples of the substituent that the arenediyl group represented by L ^11a may have include a halogen atom, a hydroxy group, a carboxy group, a cyano group, a nitro group, an alkyl group, a fluorinated alkyl group, an alkoxycarbonyloxy group, an acyl group, an acyloxy group, and an alkoxy group.

Examples of the monovalent hydrocarbon group having 1 to 20 carbon atoms represented by R ¹⁸ include a monovalent chain hydrocarbon group having 1 to 10 carbon atoms, a monovalent alicyclic hydrocarbon group having 3 to 20 carbon atoms, and a monovalent aromatic hydrocarbon group having 6 to 20 carbon atoms.

Examples of the monovalent chain hydrocarbon group having 1 to 10 carbon atoms represented by R ¹⁸ to R ²⁰ include a monovalent linear or branched saturated hydrocarbon group having 1 to 10 carbon atoms, or a monovalent linear or branched unsaturated hydrocarbon group having 1 to 10 carbon atoms.

As the monovalent alicyclic hydrocarbon group having 3 to 20 carbon atoms represented by R ¹⁸ to R ²⁰ , the monovalent alicyclic hydrocarbon group having 3 to 20 carbon atoms represented by R ² in the above formula (1) can be suitably used.

As the monovalent aromatic hydrocarbon group having 6 to 20 carbon atoms represented by R ¹⁸ , the monovalent aromatic hydrocarbon group having 6 to 20 carbon atoms represented by R ² in the above formula (1) can be suitably used.

R ¹⁸ is preferably a linear or branched saturated hydrocarbon group having 1 to 10 carbon atoms or an alicyclic hydrocarbon group having 3 to 20 carbon atoms.

The divalent alicyclic group having 3 to 20 carbon atoms formed when R ¹⁹ and R ²⁰ are combined together with the carbon atom to which they are bonded can suitably be a group in which one hydrogen atom has been removed from the monovalent alicyclic hydrocarbon group having 3 to 20 carbon atoms.

Among these, it is preferred that R ¹⁸ is an alkyl group, alkenyl group, or phenyl group having 1 to 4 carbon atoms, and that the alicyclic structure formed by combining R ¹⁹ and R ²⁰ together with the carbon atoms to which they are bonded is a polycyclic or monocyclic cycloalkane structure.

As the substituents which R ¹⁸ to R ²⁰ may have, the substituents which the arenediyl group represented by L ^11a may suitably have can be used.

Examples of the structural unit (I-1) include structural units represented by the following formulas (3-1) to (3-14) (hereinafter also referred to as "structural units (I-1-1) to (I-1-14)").

In the above formulas (3-1) to (3-14), R ¹⁷ to R ²⁰ have the same meanings as in the above formula (3). R ^L11 is a halogen atom, a hydroxy group, a carboxy group, a cyano group, a nitro group, an alkyl group, a fluorinated alkyl group, an alkoxycarbonyloxy group, an acyl group, an acyloxy group, or an alkoxy group. i and j are each independently an integer of 1 to 4. k and l are 0 or 1. 3a are each independently an integer of 0 to 3. When 3a is 2 or more, multiple R ^L11 are the same or different. a4 is an integer of 1 to 3.

i and j are preferably 1. R ¹⁸ is preferably a methyl group, an ethyl group, an isopropyl group, a t-butyl group, an ethenyl group, a phenyl group, or an iodophenyl group. ^{R 19} and R ²⁰ are preferably a methyl group, an ethyl group, or an isopropyl group. R ^L11 is preferably an iodine atom, a hydroxy group, or an alkoxy group. By employing an iodine atom as ^{R L11} , an iodo group can be suitably introduced into the structural unit (I).

Furthermore, the polymer may contain structural units represented by the following formulas (1f) to (2f) as structural unit (I).

In the above formulas (1f) to (2f), R ^αf each independently represents a hydrogen atom, a fluorine atom, a methyl group, or a trifluoromethyl group. ^{R βf} each independently represents a hydrogen atom or a chain alkyl group having 1 to 5 carbon atoms. _h1 is an integer from 1 to 4.

The _above R ^βf is preferably a hydrogen atom, a methyl group or an ethyl group.

The lower limit of the content of structural unit (I) (the total content when multiple types are included) relative to all structural units constituting the base polymer is preferably 10 mol%, more preferably 20 mol%, and even more preferably 25 mol%. The upper limit of the content is preferably 80 mol%, more preferably 70 mol%, and even more preferably 65 mol%. By keeping the content of structural unit (I) within the above range, the pattern formability of the radiation-sensitive composition can be further improved.

[Structural unit (II)]
The structural unit (II) is a structural unit containing at least one selected from the group consisting of a lactone structure, a cyclic carbonate structure, and a sultone structure. By further including the structural unit (II), the base polymer can adjust its solubility in a developer, thereby improving the lithography performance, such as resolution, of the radiation-sensitive composition. Furthermore, the adhesion between a resist pattern formed from the base polymer and a substrate can be improved.

Examples of structural unit (II) include structural units represented by the following formulas (T-1) to (T-11).

In the above formula, R ^L1 is a hydrogen atom, a fluorine atom, a methyl group, or a trifluoromethyl group. R ^L2 to R ^L5 are each independently a hydrogen atom, an alkyl group having 1 to 4 carbon atoms, a cyano group, a trifluoromethyl group, a methoxy group, a methoxycarbonyl group, a hydroxy group, a hydroxymethyl group, a dimethylamino group, or a methylcyclopentyloxycarbonyl group. ^{R L4} and R ^L5 may be combined with each other to form a divalent alicyclic group having 3 to 8 carbon atoms together with the carbon atoms to which they are bonded. ^L2 is a single bond or a divalent linking group. X is an oxygen atom or a methylene group. d is an integer of 0 to 3. e is an integer of 1 to 3.

Examples of the divalent alicyclic group having 3 to 8 carbon atoms formed when R ^L4 and R ^L5 are combined together with the carbon atoms to which they are bonded include divalent alicyclic groups having 3 to 20 carbon atoms formed when R ¹⁹ and R ²⁰ in the above formula (3) are combined together with the carbon atoms to which they are bonded, and include groups having 3 to 8 carbon atoms. One or more hydrogen atoms on this alicyclic group may be substituted with a hydroxy group.

Examples of the divalent linking group represented by ^L2 above include a divalent linear or branched hydrocarbon group having 1 to 10 carbon atoms, a divalent alicyclic hydrocarbon group having 4 to 12 carbon atoms, or a group composed of one or more of these hydrocarbon groups and at least one group selected from -CO-, -O-, -NH-, and -S-.

Among these, structural units (II) are preferably structural units containing a lactone structure, more preferably structural units containing a norbornane lactone structure, and even more preferably structural units derived from norbornane lactone-yl (meth)acrylate.

The lower limit of the content of structural unit (II) (the total content when multiple types are included) relative to all structural units constituting the base polymer is preferably 2 mol%, more preferably 5 mol%, and even more preferably 8 mol%. The upper limit of the content is preferably 90 mol%, more preferably 80 mol%, and even more preferably 75 mol%. By ensuring that the content of structural unit (II) falls within the above range, the radiation-sensitive composition can further improve lithography performance such as resolution, and the adhesion of the formed resist pattern to the substrate.

[Structural unit (III)]
The base polymer optionally has other structural units in addition to the structural units (I) and (II). Examples of the other structural units include a structural unit (III) containing a polar group (excluding those corresponding to the structural units (I) and (II)). By further including the structural unit (III), the base polymer can adjust its solubility in a developer, thereby improving the lithography performance, such as the resolution, of the radiation-sensitive composition. Examples of the polar group include a hydroxy group, a carboxy group, a cyano group, a nitro group, and a sulfonamide group. Among these, a hydroxy group and a carboxy group are preferred, and a hydroxy group is more preferred.

Examples of structural unit (III) include structural units represented by the following formula:

In the above formula, R 1 ^K is a hydrogen atom, a fluorine atom, a methyl group, or a trifluoromethyl group.

When the base polymer contains the structural unit (III) having the polar group, the lower limit of the content of the structural unit (III) (the total content when multiple types are included) is preferably 1 mol %, more preferably 2 mol %, and even more preferably 3 mol %, based on all structural units constituting the base polymer. The upper limit of the content is preferably 30 mol %, more preferably 20 mol %, and even more preferably 15 mol %. By ensuring that the content of the structural unit (III) falls within the above range, the lithography performance, such as resolution, of the radiation-sensitive composition can be further improved.

[Structural unit (IV)]
The base polymer optionally contains, as other structural units, a structural unit having a phenolic hydroxyl group (hereinafter also referred to as "structural unit (IV)") in addition to the structural unit (III) having the polar group. The structural unit (IV) contributes to improving etching resistance and improving the difference in developer solubility (dissolution contrast) between exposed and unexposed areas. The polymer is suitable for pattern formation using exposure to radiation having a wavelength of 50 nm or less, such as a KrF excimer laser, electron beam, or EUV. In this case, the polymer preferably contains the structural unit (I) in addition to the structural unit (IV).

The structural unit having a phenolic hydroxyl group is preferably represented by the following formula (4):

(In the above formula (4),
R ^β is a hydrogen atom, a fluorine atom, a methyl group or a trifluoromethyl group.
L ^CA is a single bond, —COO— ^* or —O—. * is a bond on the aromatic ring side.
R ¹⁰² is a halogen atom, a cyano group, a nitro group, an alkyl group, an alkoxycarbonyl group, an acyl group or an acyloxy group. When a plurality of R ¹⁰² are present, the plurality of R ¹⁰² may be the same or different.
_n3 is an integer of 0 to 2, _m3 is an integer of 1 to 8, and each _m4 is independently an integer of 0 to 8, provided that 1≦ _m3 + _m4 ≦ _2n3 +5 is satisfied.

From the viewpoint of copolymerizability of the monomer that gives the structural unit (IV), R ^β is preferably a hydrogen atom or a methyl group.

^LCA is preferably a single bond or -COO- ^* .

The halogen atom in R ¹⁰² is preferably an iodine atom.

The above _n3 is more preferably 0 or 1, and even more preferably 0.

The above _m3 is preferably an integer of 1 to 3, more preferably 1 or 2.

The above _m4 is preferably an integer of 0 to 3, and more preferably an integer of 0 to 2.

When obtaining structural unit (IV), it is preferable to polymerize the corresponding monomer in a state in which the phenolic hydroxyl group is protected with a protecting group such as an alkali-dissociable group (e.g., an acyl group), and then to obtain structural unit (IV) by deprotecting the monomer through hydrolysis. Polymerization of the monomer may also be carried out without protecting the phenolic hydroxyl group.

In the case of polymers intended for exposure to KrF excimer lasers or radiation having a wavelength of 50 nm or less, the lower limit of the content of structural unit (IV) (the total content if multiple types are included) is preferably 20 mol%, more preferably 40 mol%, based on all structural units constituting the base polymer. The upper limit of this content is preferably 60 mol%, more preferably 50 mol%.

[Other structural units]
The base polymer may contain a structural unit having an alicyclic structure represented by the following formula (6) (hereinafter also referred to as "structural unit (VII)") as a structural unit other than the structural units listed above.

(In the above formula (6), R ^1α is a hydrogen atom, a fluorine atom, a methyl group, or a trifluoromethyl group. R ^2α is a monovalent alicyclic hydrocarbon group having 3 to 20 carbon atoms.)

In the above formula (6), as the monovalent alicyclic hydrocarbon group having 3 to 20 carbon atoms represented by R ^2α , the monovalent alicyclic hydrocarbon group having 3 to 20 carbon atoms represented by R ² in the above formula (1) can be suitably used.

When the base polymer contains structural unit (VII), the lower limit of the content of structural unit (VII) is preferably 2 mol%, more preferably 5 mol%, and even more preferably 8 mol%, based on all structural units constituting the base polymer. The upper limit of the content is preferably 30 mol%, more preferably 20 mol%, and even more preferably 15 mol%.

(Method for synthesizing base polymer)
The base polymer can be synthesized, for example, by polymerizing the monomers that provide the respective structural units in an appropriate solvent using a radical polymerization initiator or the like.

The above-mentioned radical polymerization initiators include azo-based radical initiators such as azobisisobutyronitrile (AIBN), 2,2'-azobis(4-methoxy-2,4-dimethylvaleronitrile), 2,2'-azobis(2-cyclopropylpropionitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), and dimethyl 2,2'-azobisisobutyrate; and peroxide-based radical initiators such as benzoyl peroxide, t-butyl hydroperoxide, and cumene hydroperoxide. Of these, AIBN and dimethyl 2,2'-azobisisobutyrate are preferred, with AIBN being more preferred. These radical initiators can be used alone or in combination of two or more.

Examples of the solvent used in the polymerization include alkanes such as n-pentane, n-hexane, n-heptane, n-octane, n-nonane, and n-decane;
cycloalkanes such as cyclohexane, cycloheptane, cyclooctane, decalin, and norbornane;
aromatic hydrocarbons such as benzene, toluene, xylene, ethylbenzene, and cumene;
Halogenated hydrocarbons such as chlorobutanes, bromohexanes, dichloroethanes, hexamethylene dibromide, and chlorobenzene;
saturated carboxylic acid esters such as ethyl acetate, n-butyl acetate, isobutyl acetate, methyl propionate, and propylene glycol monomethyl ether acetate;
Ketones such as acetone, methyl ethyl ketone, 2-butanone, 4-methyl-2-pentanone, 2-heptanone, and cyclohexanone;
ethers such as tetrahydrofuran, dimethoxyethanes, diethoxyethane, and 1,4-dioxanes;
Alcohols such as methanol, ethanol, 1-propanol, 2-propanol, 4-methyl-2-pentanol, and 1-methoxy-2-propanol (propylene glycol monomethyl ether);
Examples of the solvents used in the polymerization include lactones such as γ-butyrolactone, etc. These solvents may be used alone or in combination of two or more.

The reaction temperature for the above polymerization is typically 40°C to 150°C, with 50°C to 120°C being preferred. The reaction time is typically 1 hour to 48 hours, with 1 hour to 24 hours being preferred.

The molecular weight of the base polymer is not particularly limited, but the lower limit of the polystyrene equivalent weight average molecular weight (Mw) measured by gel permeation chromatography (GPC) is preferably 3,000, more preferably 4,000, and even more preferably 5,000. The upper limit of Mw is preferably 30,000, more preferably 20,000, and even more preferably 12,000. By ensuring that the Mw of the base polymer falls within the above range, the resulting resist film can have good heat resistance and developability.

The ratio of Mw to the polystyrene-equivalent number average molecular weight (Mn) of the base polymer (Mw/Mn) measured by GPC is typically 1 or more and 5 or less, preferably 1 or more and 3 or less, and more preferably 1 or more and 2 or less.

The Mw and Mn of the polymers herein are values measured using gel permeation chromatography (GPC) under the following conditions:

GPC columns: 2 G2000HXL, 1 G3000HXL, 1 G4000HXL (all manufactured by Tosoh)
Column temperature: 40°C
Elution solvent: tetrahydrofuran Flow rate: 1.0 mL/min Sample concentration: 1.0 mass%
Sample injection volume: 100 μL
Detector: Differential refractometer Standard material: Monodisperse polystyrene

The content of the base polymer is preferably 40% by mass or more, more preferably 50% by mass or more, and even more preferably 55% by mass or more, based on the total solids content of the radiation-sensitive composition.

(Other polymers)
The radiation-sensitive composition of this embodiment may contain, as the other polymer, a polymer having a higher mass content of fluorine atoms than the base polymer (hereinafter also referred to as a "high-fluorine content polymer." When the radiation-sensitive composition contains a high-fluorine content polymer, the high-fluorine content polymer can be unevenly distributed in the surface layer of the resist film relative to the base polymer, thereby improving the water repellency of the surface of the resist film during immersion exposure, and controlling the surface modification of the resist film and the distribution of composition within the film during EUV exposure.

The high-fluorine content polymer may have, for example, a structural unit represented by the following formula (5) (hereinafter also referred to as "structural unit (V)").

In the above formula (5), R ¹³ is a hydrogen atom, a methyl group, or a trifluoromethyl group. G ^L is a single bond, an alkanediyl group having 1 to 5 carbon atoms, an oxygen atom, a sulfur atom, -COO-, -SO ₂ ONH-, -CONH-, -OCONH-, or a combination thereof. R ¹⁴ is a monovalent fluorinated chain hydrocarbon group having 1 to 20 carbon atoms or a monovalent fluorinated alicyclic hydrocarbon group having 3 to 20 carbon atoms.

From the viewpoint of copolymerizability of the monomer that gives the structural unit (V), R ¹³ is preferably a hydrogen atom or a methyl group, more preferably a methyl group.

As the above G ^L , from the viewpoint of copolymerizability of the monomer that gives the structural unit (V), a single bond and -COO- are preferred, and -COO- is more preferred.

Examples of the monovalent fluorinated chain hydrocarbon group having 1 to 20 carbon atoms represented by R ¹⁴ include linear or branched alkyl groups having 1 to 20 carbon atoms in which some or all of the hydrogen atoms have been substituted with fluorine atoms.

Examples of the monovalent fluorinated alicyclic hydrocarbon group having 3 to 20 carbon atoms represented by R ¹⁴ include monocyclic or polycyclic hydrocarbon groups having 3 to 20 carbon atoms in which some or all of the hydrogen atoms have been substituted with fluorine atoms.

R ¹⁴ is preferably a fluorinated chain hydrocarbon group, more preferably a fluorinated alkyl group, and even more preferably a 2,2,2-trifluoroethyl group, a 2,2,3,3,3-pentafluoropropyl group, a 1,1,1,3,3,3-hexafluoropropyl group, or a 5,5,5-trifluoro-1,1-diethylpentyl group.

When the high-fluorine content polymer has structural unit (V), the lower limit of the content of structural unit (V) is preferably 50 mol%, more preferably 60 mol%, and even more preferably 70 mol%, relative to all structural units constituting the high-fluorine content polymer. The upper limit of this content is preferably 95 mol%, more preferably 90 mol%, and even more preferably 85 mol%. By setting the content of structural unit (V) within the above range, the mass content of fluorine atoms in the high-fluorine content polymer can be more appropriately adjusted, further promoting uneven distribution of fluorine atoms in the surface layer of the resist film, and as a result, the water repellency of the resist film during immersion exposure can be further improved.

The high-fluorine content polymer may have a fluorine atom-containing structural unit represented by the following formula (f-2) (hereinafter also referred to as structural unit (VI)) in addition to or instead of the structural unit (V). By having the structural unit (f-2), the high-fluorine content polymer has improved solubility in alkaline developers, making it possible to suppress the occurrence of development defects.

Structural unit (VI) can be broadly classified into two types: (x) a unit having an alkali-soluble group; and (y) a unit having a group that dissociates under the action of an alkali to increase solubility in an alkaline developer (hereinafter simply referred to as an "alkali-dissociable group"). Common to both (x) and (y), in the above formula (f-2), R ^C is a hydrogen atom, a fluorine atom, a methyl group, or a trifluoromethyl group. R ^D is a single bond, an (s+1)-valent hydrocarbon group having 1 to 20 carbon atoms, a structure in which an oxygen atom, a sulfur atom, -NR ^dd -, a carbonyl group, -COO-, -OCO-, or -CONH- is bonded to the R ^E terminal of this hydrocarbon group, or a structure in which some of the hydrogen atoms of this hydrocarbon group are substituted with an organic group having a heteroatom. R ^dd is a hydrogen atom or a monovalent hydrocarbon group having 1 to 10 carbon atoms. s is an integer from 1 to 3.

When the structural unit (VI) has (x) an alkali-soluble group, R ^F is a hydrogen atom, and A ¹ is an oxygen atom, -COO-*, or -SO ₂ O-*. * indicates the site bonding to R ^{F. W 1} is a single bond, a hydrocarbon group of 1 to 20 carbon atoms, or a divalent fluorinated hydrocarbon group. When ^{A 1} is an oxygen atom, W ¹ is a fluorinated hydrocarbon group having a fluorine atom or a fluoroalkyl group on the carbon atom to which A ¹ is bonded. R ^E is a single bond or a divalent organic group of 1 to 20 carbon atoms. When s is 2 or 3, multiple R ^{E s} , W ^{1 s} , A ^{1 s} , and R ^F s may be the same or different. When the structural unit (VI) has (x) an alkali-soluble group, it is possible to increase the affinity for an alkaline developer and suppress development defects. (x) As the structural unit (VI) having an alkali-soluble group, it is particularly preferred that A ¹ is an oxygen atom and W ¹ is a 1,1,1,3,3,3-hexafluoro-2,2-methanediyl group.

When the structural unit (VI) has an alkali-dissociable group (y), ^RF is a monovalent organic group having 1 to 30 carbon atoms, and ^A1 is an oxygen atom, -NR ^aa -, -COO-*, -OCO-*, or -SO ₂ O-*. R ^aa is a hydrogen atom or a monovalent hydrocarbon group having 1 to 10 carbon atoms. * indicates the bonding site to ^RF . ^{W 1} is a single bond or a divalent fluorinated hydrocarbon group having 1 to 20 carbon atoms. ^{R E} is a single bond or a divalent organic group having 1 to 20 carbon atoms. When ^{A 1} is -COO-*, -OCO-*, or -SO ₂ O-*, W ¹ or ^RF has a fluorine atom on the carbon atom bonding to A ¹ or on the carbon atom adjacent thereto. When A ¹ is an oxygen atom, W ¹ and R ^E are single bonds, R ^D is a structure in which a carbonyl group is bonded to the R ^E terminal of a hydrocarbon group having 1 to 20 carbon atoms, and R ^F is an organic group having a fluorine atom. When s is 2 or 3, multiple R ^{E s} , W ^{1 s} , A ^{1 s} , and R ^{F s} may be the same or different. When the structural unit (VI) has (y) an alkali-dissociable group, the resist film surface changes from hydrophobic to hydrophilic in the alkaline development step. As a result, affinity for the developer is significantly increased, and development defects can be more efficiently suppressed. As the structural unit (VI) having (y) an alkali-dissociable group, one in which A ¹ is -COO-* and R ^F or W ¹ or both have a fluorine atom is particularly preferred.

As R 3 ^C , from the viewpoint of copolymerizability of the monomer that gives the structural unit (VI), a hydrogen atom or a methyl group is preferred, and a methyl group is more preferred.

When the high-fluorine content polymer has structural unit (VI), the content of structural unit (VI) is preferably 30 mol%, more preferably 40 mol%, and even more preferably 50 mol%, based on all structural units constituting the high-fluorine content polymer. The upper limit of this content is preferably 95 mol%, more preferably 90 mol%, and even more preferably 80 mol%. By ensuring that the content of structural unit (VI) falls within this range, it is possible to increase the water repellency of the resist film during immersion exposure and to improve its solubility in an alkaline developer, thereby suppressing the occurrence of development defects.

[Other structural units]
The high fluorine content polymer may, if necessary, contain structural units other than the structural units listed above, such as structural unit (I), structural unit (III) or structural unit (VII) in the base polymer.

When the high-fluorine content polymer contains structural unit (I), the content of structural unit (I) is preferably 5 mol %, more preferably 8 mol %, based on all structural units constituting the high-fluorine content polymer. The upper limit of this content is preferably 40 mol %, more preferably 30 mol %.

When the high-fluorine content polymer contains structural unit (III), the content of structural unit (III) is preferably 5 mol %, more preferably 8 mol %, based on all structural units constituting the high-fluorine content polymer. The upper limit of the content is preferably 50 mol %, more preferably 40 mol %.

When the high-fluorine content polymer contains structural unit (VII), the content of structural unit (VII) is preferably 20 mol %, more preferably 30 mol %, based on all structural units constituting the high-fluorine content polymer. The upper limit of this content is preferably 60 mol %, more preferably 50 mol %.

The lower limit of the Mw of the high-fluorine content polymer is preferably 3,000, more preferably 4,000, and even more preferably 5,000. The upper limit of the Mw is preferably 20,000, more preferably 10,000, and even more preferably 8,000.

The lower limit of Mw/Mn for high fluorine content polymers is typically 1, and more preferably 1.1. The upper limit of Mw/Mn is typically 5, and preferably 3, and more preferably 2.

When the radiation-sensitive composition contains a high-fluorine-content polymer, the lower limit of the content of the high-fluorine-content polymer is preferably 0.5 parts by mass, more preferably 1 part by mass, and even more preferably 1.5 parts by mass, per 100 parts by mass of the base polymer. The upper limit of the content is preferably 15 parts by mass, more preferably 10 parts by mass, and even more preferably 8 parts by mass.

By setting the content of the high-fluorine content polymer within the above range, the high-fluorine content polymer can be more effectively distributed unevenly on the surface layer of the resist film, which in turn improves the water repellency of the surface of the resist film during immersion exposure, and allows for surface modification of the resist film during EUV exposure and control of the distribution of composition within the film. The radiation-sensitive composition may contain one or more types of high-fluorine content polymer.

(Method for synthesizing high fluorine content polymer)
The high fluorine content polymer can be synthesized by the same method as the above-mentioned method for synthesizing the base polymer.

(solvent)
The radiation-sensitive composition according to this embodiment contains a solvent. The solvent is not particularly limited as long as it can dissolve or disperse at least the onium salt compound (1), the base polymer, and optional components that may be contained as desired.

Examples of solvents include alcohol-based solvents, ether-based solvents, ketone-based solvents, amide-based solvents, ester-based solvents, and hydrocarbon-based solvents.

Examples of alcohol-based solvents include:
monoalcohol solvents having 1 to 18 carbon atoms, such as isopropanol, 4-methyl-2-pentanol, 3-methoxybutanol, n-hexanol, 2-ethylhexanol, furfuryl alcohol, cyclohexanol, 3,3,5-trimethylcyclohexanol, and diacetone alcohol;
polyhydric alcohol solvents having 2 to 18 carbon atoms, such as ethylene glycol, 1,2-propylene glycol, 2-methyl-2,4-pentanediol, 2,5-hexanediol, diethylene glycol, dipropylene glycol, triethylene glycol, and tripropylene glycol;
Examples of suitable polyhydric alcohol solvents include partially etherified polyhydric alcohol solvents in which some of the hydroxy groups of the above polyhydric alcohol solvents have been etherified.
In this embodiment, alcoholic acid ester solvents such as methyl lactate, ethyl lactate, propyl lactate, butyl lactate, methyl 2-hydroxyisobutyrate, isopropyl 2-hydroxyisobutyrate, isobutyl 2-hydroxyisobutyrate, and n-butyl 2-hydroxyisobutyrate are also included in the alcoholic solvents.

Examples of ether solvents include:
dialkyl ether solvents such as diethyl ether, dipropyl ether, and dibutyl ether;
cyclic ether solvents such as tetrahydrofuran and tetrahydropyran;
aromatic ring-containing ether solvents such as diphenyl ether and anisole (methyl phenyl ether);
Examples of the polyhydric alcohol solvent include polyhydric alcohol ether solvents obtained by etherifying the hydroxy groups of the above polyhydric alcohol solvents.

Examples of the ketone solvent include chain ketone solvents such as acetone, butanone, and methyl isobutyl ketone:
Cyclic ketone solvents such as cyclopentanone, cyclohexanone, and methylcyclohexanone:
Examples include 2,4-pentanedione, acetonylacetone, and acetophenone.

Examples of the amide solvent include cyclic amide solvents such as N,N'-dimethylimidazolidinone and N-methylpyrrolidone;
Examples of the solvent include chain amide solvents such as N-methylformamide, N,N-dimethylformamide, N,N-diethylformamide, acetamide, N-methylacetamide, N,N-dimethylacetamide, and N-methylpropionamide.

Examples of ester solvents include:
Monocarboxylic acid ester solvents such as n-butyl acetate;
polyhydric alcohol partial ether acetate solvents such as diethylene glycol mono-n-butyl ether acetate, propylene glycol monomethyl ether acetate, and dipropylene glycol monomethyl ether acetate;
Lactone solvents such as γ-butyrolactone and valerolactone;
Carbonate solvents such as diethyl carbonate, ethylene carbonate, and propylene carbonate;
Examples of the solvent include polycarboxylic acid diester solvents such as propylene glycol diacetate, methoxytriglyceride acetate, diethyl oxalate, ethyl acetoacetate, and diethyl phthalate.

Examples of hydrocarbon solvents include aliphatic hydrocarbon solvents such as n-hexane, cyclohexane, and methylcyclohexane;
Examples of the solvent include aromatic hydrocarbon solvents such as benzene, toluene, diisopropylbenzene, and n-amylnaphthalene.

Among these, alcohol-based solvents, ester-based solvents, and ketone-based solvents are preferred, with polyhydric alcohol partial ether acetate-based solvents, polyhydric alcohol partial ether-based solvents, lactone-based solvents, and cyclic ketone-based solvents being more preferred, and propylene glycol monomethyl ether acetate, propylene glycol monomethyl ether, γ-butyrolactone, and cyclohexanone being even more preferred. The radiation-sensitive composition may contain one or more solvents.

(Other optional ingredients)
The radiation-sensitive composition may contain other optional components in addition to the above components. Examples of the other optional components include a crosslinking agent, a localization promoter, a surfactant, an alicyclic skeleton-containing compound, and a sensitizer. These other optional components may be used alone or in combination of two or more.

<Method for preparing radiation-sensitive composition>
The radiation-sensitive composition can be prepared, for example, by mixing the onium salt compound (1), the polymer, and, if necessary, a high-fluorine-content polymer, and a solvent in a predetermined ratio. After mixing, the radiation-sensitive composition is preferably filtered, for example, through a filter having a pore size of about 0.05 μm to 0.40 μm. The solids concentration of the radiation-sensitive composition is usually 0.1% to 50% by mass, preferably 0.5% to 30% by mass, and more preferably 1% to 20% by mass.

<Pattern Forming Method>
A pattern forming method according to one embodiment of the present invention includes:
a step (1) of forming a resist film by directly or indirectly applying the radiation-sensitive composition to a substrate (hereinafter also referred to as a "resist film forming step");
a step (2) of exposing the resist film to light (hereinafter also referred to as an "exposure step");
The method includes a step (3) of developing the exposed resist film with a developer (hereinafter also referred to as the "developing step").

The above-described pattern formation method uses the above-described radiation-sensitive composition, which is capable of exhibiting excellent sensitivity, CDU, MEEF, development defect suppression, pattern circularity, and pattern rectangularity during pattern formation, making it possible to efficiently form high-quality resist patterns. Each step is described below.

[Resist film forming process]
In this step (step (1) above), a resist film is formed from the radiation-sensitive composition. Examples of substrates on which this resist film is formed include conventionally known substrates such as silicon wafers, silicon dioxide wafers, and aluminum-coated wafers. Alternatively, an organic or inorganic anti-reflective coating, such as those disclosed in JP-B-6-12452 and JP-A-59-93448, may be formed on the substrate. Examples of coating methods include spin coating, casting coating, and roll coating. After coating, if necessary, pre-baking (PB) may be performed to volatilize the solvent in the coating film. The PB temperature is typically 60°C to 160°C, preferably 80°C to 140°C. The PB time is typically 5 to 600 seconds, preferably 10 to 300 seconds.

The lower limit of the thickness of the resist film formed is preferably 10 nm, more preferably 15 nm, and even more preferably 20 nm. The upper limit of the thickness is preferably 500 nm, more preferably 350 nm, and even more preferably 280 nm.

When performing immersion exposure, regardless of whether the radiation-sensitive composition contains a water-repellent polymer additive such as the high-fluorine content polymer, an immersion protective film that is insoluble in the immersion fluid may be provided on the formed resist film to prevent direct contact between the immersion fluid and the resist film. The immersion protective film may be either a solvent-removable protective film that is removed with a solvent before the development step (see, for example, JP-A 2006-227632), or a developer-removable protective film that is removed simultaneously with development in the development step (see, for example, WO 2005-069076 and WO 2006-035790). However, from the perspective of throughput, it is preferable to use a developer-removable immersion protective film.

[Exposure process]
In this step (step (2) above), the resist film formed in the resist film formation step (1) above is exposed to radiation through a photomask (or, in some cases, through an immersion liquid such as water). Examples of radiation used for exposure include electromagnetic waves such as visible light, ultraviolet light, far ultraviolet light, EUV (extreme ultraviolet), X-rays, and gamma rays; and charged particle beams such as electron beams and alpha rays, depending on the line width of the desired pattern. Among these, far ultraviolet light, electron beams, and EUV are preferred, with ArF excimer laser light (wavelength 193 nm), KrF excimer laser light (wavelength 248 nm), electron beams, and EUV being more preferred, and electron beams and EUV with wavelengths of 50 nm or less, which are positioned as next-generation exposure technologies, being even more preferred.

When exposure is performed by immersion exposure, the immersion liquid used can be, for example, water or a fluorine-based inert liquid. The immersion liquid is preferably a liquid that is transparent to the exposure wavelength and has as small a temperature coefficient of refractive index as possible to minimize distortion of the optical image projected onto the film. However, particularly when the exposure light source is an ArF excimer laser (wavelength 193 nm), water is preferred for its ease of availability and handling, in addition to the above considerations. When using water, a small proportion of an additive that reduces the surface tension of the water and increases its surfactant power may be added. It is preferable that this additive does not dissolve the resist film on the wafer and has a negligible effect on the optical coating on the underside of the lens. Distilled water is preferred.

After the exposure, it is preferable to perform a post-exposure bake (PEB) to promote dissociation of acid-dissociable groups in the polymer, etc., in the exposed areas of the resist film due to the acid generated from the radiation-sensitive acid generator upon exposure. This PEB creates a difference in solubility in the developer between the exposed and unexposed areas. The PEB temperature is typically 50°C to 180°C, with 80°C to 130°C being preferred. The PEB time is typically 5 to 600 seconds, with 10 to 300 seconds being preferred.

[Development process]
In this step (step (3) above), the resist film exposed in the exposure step (step (2) above) is developed. This allows a predetermined resist pattern to be formed. After development, the resist film is generally washed with a rinse liquid such as water or alcohol, and then dried.

In the case of alkaline development, the developer used for the above development may, for example, be an alkaline aqueous solution containing at least one alkaline compound such as sodium hydroxide, potassium hydroxide, sodium carbonate, sodium silicate, sodium metasilicate, aqueous ammonia, ethylamine, n-propylamine, diethylamine, di-n-propylamine, triethylamine, methyldiethylamine, ethyldimethylamine, triethanolamine, tetramethylammonium hydroxide (TMAH), pyrrole, piperidine, choline, 1,8-diazabicyclo-[5.4.0]-7-undecene, or 1,5-diazabicyclo-[4.3.0]-5-nonene. Of these, a TMAH aqueous solution is preferred, with a 2.38% by mass TMAH aqueous solution being more preferred.

In the case of organic solvent development, examples of the organic solvent include hydrocarbon solvents, ether solvents, ester solvents, ketone solvents, and alcohol solvents, as well as solvents containing an organic solvent. Examples of the organic solvent include one or more of the solvents listed above as solvents for the radiation-sensitive composition. Among these, ether solvents, ester solvents, and ketone solvents are preferred. As ether solvents, glycol ether solvents are preferred, with ethylene glycol monomethyl ether and propylene glycol monomethyl ether being more preferred. As ester solvents, acetate ester solvents are preferred, with n-butyl acetate and amyl acetate being more preferred. As ketone solvents, chain ketones are preferred, with 2-heptanone being more preferred. The content of the organic solvent in the developer is preferably 80% by mass or more, more preferably 90% by mass or more, even more preferably 95% by mass or more, and particularly preferably 99% by mass or more. Examples of components other than the organic solvent in the developer include water and silicone oil.

As mentioned above, the developer may be either an alkaline developer or an organic solvent developer. It can be selected appropriately depending on whether the desired pattern is a positive or negative one.

Development methods include, for example, immersing the substrate in a tank filled with developer for a certain period of time (dip method), puddling the developer on the surface of the substrate using surface tension and leaving it to stand for a certain period of time (puddle method), spraying the developer onto the substrate surface (spray method), and continuously dispensing developer while scanning a developer dispensing nozzle at a constant speed over a substrate that is rotating at a constant speed (dynamic dispense method).

<Onium Salt Compound>
The onium salt compound is represented by the following formula (1a): As such an onium salt compound, onium salt compound (1) can be suitably used, except that in formula (1) in the radiation-sensitive composition, the monovalent organic group of R ¹ in the organic acid anion has 5 to 40 carbon atoms.

(In formula (1a),
R ^1a is a monovalent organic group having 5 to 40 carbon atoms.
A ⁻ is —SO ₃ ⁻ , —COO ^— or —N ⁻ —SO ₂ —R ^X. R ^X is a monovalent organic group having 1 to 20 carbon atoms.
E ¹ is —O—, —S—, —SO— or —SO ₂ —.
^R2 is a hydrogen atom or a monovalent organic group having 1 to 20 carbon atoms.
E is —O— or —NR ^Y —, and ^{R Y} is a hydrogen atom or a monovalent hydrocarbon group having 1 to 10 carbon atoms.
^R3 is a hydrogen atom or a monovalent organic group having 1 to 20 carbon atoms.
^R4 and ^R5 are each independently a monovalent organic group having 1 to 20 carbon atoms, or ^R4 and ^R5 are combined with each other to form a ring structure having 4 to 12 carbon atoms together with the sulfur atom to which they are bonded, provided that in the ring structure, the ring containing the sulfur atom in formula (1a) is fused with two other rings to form a tricyclic structure, and when the ring containing the sulfur atom in formula (1a) contains a heteroatom other than the sulfur atom, the heteroatom is an oxygen atom or a nitrogen atom.
^R6 is a halogen atom, a hydroxy group, a nitro group, an amino group, a carboxy group, a cyano group, or a monovalent organic group having 1 to 20 carbon atoms. When a plurality of ^R6s are present, the plurality of ^R6s may be the same or different.
m is 0 or 1. When m is 1, both R ³ -E-CO- and R ² -E ¹ - are bonded to the 6-membered ring structure to which the sulfur atom in the above formula (1a) is bonded.
n is an integer from 0 to 4.

The present invention will be explained in detail below based on examples, but the present invention is not limited to these examples. The methods for measuring various physical properties are shown below.

[Weight average molecular weight (Mw) and number average molecular weight (Mn)]
The Mw and Mn of the polymer were measured under the conditions described above, and the dispersity (Mw/Mn) was calculated from the measurement results of Mw and Mn.

[ ^13C -NMR analysis]
The ¹³ C-NMR analysis of the polymer was carried out using a nuclear magnetic resonance spectrometer (JNM-Delta400 manufactured by JEOL Ltd.).

<Synthesis of Polymer>
The monomers used in the synthesis of each polymer in each Example and Comparative Example are shown below. In the following synthesis examples, unless otherwise specified, parts by mass refer to a value where the total mass of the monomers used is taken as 100 parts by mass, and mol % refers to a value where the total number of moles of the monomers used is taken as 100 mol %.

[Synthesis Example 1]
(Synthesis of Polymer (A-1))
Monomer (M-1), monomer (M-2), monomer (M-5), monomer (M-10), and monomer (M-14) were dissolved in 2-butanone (200 parts by mass) to a molar ratio of 40/10/20/25/5 (mol%), and AIBN (azobisisobutyronitrile) (5 mol% relative to 100 mol% of the total monomers used) was added as an initiator to prepare a monomer solution. 2-butanone (100 parts by mass) was placed in a reaction vessel, and after purging with nitrogen for 30 minutes, the reaction vessel was heated to 80°C, and the monomer solution was added dropwise over 3 hours with stirring. The start of the dropwise addition marked the start of the polymerization reaction, and the polymerization reaction was carried out for 6 hours. After completion of the polymerization reaction, the polymerization solution was cooled to below 30°C with water. The cooled polymerization solution was poured into methanol (2,000 parts by mass), and the precipitated white powder was filtered off. The filtered white powder was washed twice with methanol, filtered, and dried at 50°C for 24 hours to obtain a white powdery polymer (A-1) (yield: 80%). The Mw of the polymer (A-1) was 5,900, and the Mw/Mn was 1.61. Furthermore, as a result of ^13C -NMR analysis, the contents of the structural units derived from (M-1), (M-2), (M-5), (M-10), and (M-14) were 39.3 mol%, 9.4 mol%, 20.5 mol%, 24.8 mol%, and 6.0 mol%, respectively.

[Synthesis Examples 2 to 11]
(Synthesis of Polymers (A-2) to (A-11))
Polymers (A-2) to (A-11) were synthesized in the same manner as in Synthesis Example 1, except that the types and blending ratios of monomers shown in Table 1 below were used. The content (mol %) of each structural unit and physical properties (Mw and Mw/Mn) of the resulting polymers are also shown in Table 1 below. In Table 1 below, "-" indicates that the corresponding monomer was not used (the same applies to the following tables).

[Synthesis Example 12]
(Synthesis of Polymer (A-12))
Monomer (M-1) and monomer (M-29) were dissolved in 1-methoxy-2-propanol (200 parts by mass) to a molar ratio of 55/45 (mol%), and MAIB (dimethyl 2,2'-azobisisobutyrate) (4 mol%) was added as an initiator to prepare a monomer solution. 1-Methoxy-2-propanol (100 parts by mass) was placed in a reaction vessel, and after purging with nitrogen for 30 minutes, the reaction vessel was heated to 80°C and the monomer solution was added dropwise over 3 hours with stirring. The start of the dropwise addition marked the start of the polymerization reaction, and the polymerization reaction was carried out for 6 hours. After completion of the polymerization reaction, the polymerization solution was cooled with water to below 30°C. The cooled polymerization solution was poured into hexane (2,000 parts by mass), and the precipitated white powder was filtered off. The filtered white powder was washed twice with hexane, filtered off, and dissolved in 1-methoxy-2-propanol (300 parts by mass). Next, methanol (500 parts by mass), triethylamine (50 parts by mass), and ultrapure water (10 parts by mass) were added, and a hydrolysis reaction was carried out at 70°C for 6 hours while stirring. After completion of the reaction, the remaining solvent was distilled off, and the resulting solid was dissolved in acetone (100 parts by mass) and added dropwise to water (500 parts by mass) to coagulate the polymer. The resulting solid was filtered and dried at 50°C for 13 hours to obtain a white powdery polymer (A-12) (yield: 85%). The Mw of the polymer (A-12) was 7,000, and the Mw/Mn was 1.57. Furthermore, as a result of ^13C -NMR analysis, the content ratios of the structural units derived from (M-1) and (M-29) were 52.3 mol% and 47.8 mol%, respectively.

[Synthesis Examples 13 to 34]
(Synthesis of Polymers (A-13) to (A-34))
Polymers (A-13) to (A-34) were synthesized in the same manner as in Synthesis Example 12, except that the types and blending ratios of monomers shown in Table 2 below were used. Regarding the monomer providing structural unit (IV), ^13C -NMR measurement confirmed that the peak of the carbonyl group of the acetyl group had disappeared in the polymer, and substantially all of the alkali-dissociable groups had been hydrolyzed to phenolic hydroxyl groups. The content (mol %) of each structural unit and physical properties (Mw and Mw/Mn) of the obtained polymers are also shown in Table 2 below.

[Synthesis Example 35]
(Synthesis of High Fluorine Content Polymer (F-1))
Monomer (M-1), monomer (M-15), and monomer (M-20) were dissolved in 2-butanone (200 parts by mass) to a molar ratio of 20/10/70 (mol%), and AIBN (5 mol%) was added as an initiator to prepare a monomer solution. 2-butanone (100 parts by mass) was placed in a reaction vessel, and after purging with nitrogen for 30 minutes, the reaction vessel was heated to 80°C and the monomer solution was added dropwise over 3 hours with stirring. The start of the dropwise addition marked the start of the polymerization reaction, and the polymerization reaction was carried out for 6 hours. After completion of the polymerization reaction, the polymerization solution was cooled with water to below 30°C. The solvent was replaced with acetonitrile (400 parts by mass), and then hexane (100 parts by mass) was added, stirred, and the acetonitrile layer was collected. This process was repeated three times. The solvent was replaced with propylene glycol monomethyl ether acetate to obtain a solution of high fluorine content polymer (F-1) (yield: 75%). The high fluorine content polymer (F-1) had an Mw of 6,600 and an Mw/Mn of 1.67. As a result of C-NMR analysis, the ^contents of the structural units derived from (M-1), (M-15) and (M-20) were 19.7 mol %, 10.1 mol % and 70.2 mol %, respectively.

[Synthesis Examples 36 to 39]
(Synthesis of High Fluorine Content Polymers (F-2) to (F-5))
High fluorine content polymers (F-2) to (F-5) were synthesized in the same manner as in Synthesis Example 35, except for using monomers of the types and blending ratios shown in Table 3. The content (mol %) of each structural unit and physical properties (Mw and Mw/Mn) of the obtained high fluorine content polymers are also shown in Table 3.

<Synthesis of Radiation-Sensitive Acid Generator (B)>
[Example B1]
(Synthesis of Onium Salt Compound (B-1))
An onium salt compound (B-1) as the radiation-sensitive acid generator (B) was synthesized according to the following synthesis scheme.

20.0 mmol of diphenyl sulfoxide, 40.0 mmol of t-butyl-2-methoxybenzoate, 30.0 mmol of trifluorosulfonic anhydride (Tf ₂ O), and 50 g of dichloromethane were added to a reaction vessel and stirred at −78° C. for 12 hours. Subsequently, saturated aqueous sodium bicarbonate solution was added to the reaction solution to terminate the reaction, followed by extraction with dichloromethane and separation of the organic layer. The resulting organic layer was washed with saturated aqueous sodium chloride solution and then with water. After drying with sodium sulfate, the solvent was distilled off, and the mixture was purified by column chromatography to obtain compound (B-1-a) in good yield.

50 g of a 1 M aqueous solution of sodium iodide and 50 g of dichloromethane were added to the compound (B-1-a) and stirred at 50°C for 12 hours. Dichloromethane was then added to the reaction solution for extraction, and the organic layer was separated. The solvent in the resulting organic layer was distilled off, yielding compound (B-1-b) in good yield.

20.0 mmol of compound (B-1-c), 50 g of dichloromethane, and 50 g of water were added to the above compound (B-1-b), and the mixture was stirred at room temperature for 4 hours. Dichloromethane was added to the reaction solution for extraction, and the organic layer was separated. The resulting organic layer was washed with a saturated aqueous sodium chloride solution and then with water. After drying with sodium sulfate, the solvent was distilled off, and the mixture was purified by column chromatography to obtain compound (B-1) represented by the above formula (B-1) in good yield.

[Examples B2 to B37]
(Synthesis of onium salt compounds (B-2) to (B-37))
Onium salt compounds represented by the following formulae (B-2) to (B-37) were synthesized as radiation-sensitive acid generators in the same manner as in Example B1, except that the raw materials and precursors were changed as appropriate (onium salt compound (B-1) is also shown).

[Example C1]
(Synthesis of Onium Salt Compound (C-1))
An onium salt compound (C-1) as the acid diffusion controller (C) was synthesized according to the following synthesis scheme.

20.0 mmol of diphenyl sulfoxide, 40.0 mmol of 2-methoxybenzoic acid, 30.0 mmol of trifluorosulfonic anhydride (Tf ₂ O), and 50 g of dichloromethane were added to a reaction vessel and stirred at −78° C. for 12 hours. Water was then added to the reaction solution to terminate the reaction, followed by extraction with dichloromethane and separation of the organic layer. The resulting organic layer was washed with a saturated aqueous sodium chloride solution and then with water. After drying over sodium sulfate, the solvent was removed by distillation, and the mixture was purified by column chromatography to obtain compound (C-1-a) in good yield.

50 g of a 1 M aqueous solution of sodium iodide and 50 g of dichloromethane were added to the compound (C-1-a) and stirred at 50°C for 12 hours. Dichloromethane was then added to the reaction solution for extraction, and the organic layer was separated. The solvent in the resulting organic layer was distilled off, yielding compound (C-1-b) in good yield.

20.0 mmol of compound (C-1-c), 50 g of dichloromethane, and 50 g of water were added to the above compound (C-1-b), and the mixture was stirred at room temperature for 4 hours. Dichloromethane was added to the reaction mixture for extraction, and the organic layer was separated. The resulting organic layer was washed with a saturated aqueous sodium chloride solution and then with water. After drying over sodium sulfate, the solvent was distilled off, yielding compound (C-1) represented by the above formula (C-1) in good yield.

[Examples C2 to C17]
(Synthesis of onium salt compounds (C-2) to (C-17))
Onium salt compounds as acid diffusion controllers represented by the following formulae (C-2) to (C-17) were synthesized in the same manner as in Example C1 and Example B1, except that the raw materials and precursors were appropriately changed.

In addition to the components synthesized above, the following compounds were used:

[Radiation-sensitive acid generators other than the radiation-sensitive acid generators (B-1) to (B-37)]
b-1 to b-10: Compounds represented by the following formulas (b-1) to (b-10) (hereinafter, the compounds represented by formulas (b-1) to (b-10) may be referred to as "compound (b-1)" to "compound (b-10)," respectively.)

[Radiation-sensitive acid generators other than the acid diffusion controllers (C-1) to (C-17)]
cc-1 to cc-6: Compounds represented by the following formulas (cc-1) to (cc-6) (hereinafter, the compounds represented by formulas (cc-1) to (cc-6) may be referred to as "compound (cc-1)" to "compound (cc-6)", respectively).

[Solvent (E)]
E-1: Propylene glycol monomethyl ether acetate E-2: Propylene glycol monomethyl ether E-3: γ-butyrolactone E-4: Cyclohexanone

[Other additive components (W)]
W-1: MEGAFACE EFS-321 (manufactured by DIC Corporation) (fluorine-free)
W-2: BYK-399 (manufactured by BYK Japan Co., Ltd.) (non-silicone type)

[Preparation of Positive Radiation-Sensitive Composition for ArF Immersion Exposure]
[Example 1]
A radiation-sensitive composition (J-1) was prepared by mixing 100 parts by mass of (A-1) as the polymer (A), 10.0 parts by mass of (B-1) as the radiation-sensitive acid generator (B), 10.0 parts by mass of (cc-1) as the acid diffusion controller (C), 2.0 parts by mass (solids content) of (F-1) as the high fluorine content polymer (F), 0.1 parts by mass of (W-1) as the other additive component (W), and 3,400 parts by mass of a mixed solvent of (E-1)/(E-2)/(E-3) as the solvent (E), and filtering the mixture through a membrane filter having a pore size of 0.2 μm.

[Examples 2 to 43, 101 to 103 and Comparative Examples 1 to 13]
Radiation-sensitive compositions (J-2) to (J-43), (J-101) to (J-103), and (CJ-1) to (CJ-13) were prepared in the same manner as in Example 1, except that the types and amounts of each component shown in Tables 4-1 and 4-2 below were used.

<Formation of Resist Pattern Using Positive Radiation-Sensitive Composition for ArF Immersion Exposure>
A composition for forming a bottom antireflective coating ("ARC66" from Brewer Science) was applied to a 12-inch silicon wafer using a spin coater ("CLEAN TRACK ACT12" from Tokyo Electron Limited), and then heated at 205°C for 60 seconds to form a bottom antireflective coating with an average thickness of 100 nm. The positive radiation-sensitive composition for ArF exposure prepared above was applied to this bottom antireflective coating using the spin coater, and prebaked at 100°C for 60 seconds. This was followed by cooling at 23°C for 30 seconds to form a resist film with an average thickness of 120 nm. Next, this resist film was exposed to light using an ArF excimer laser immersion exposure system (ASML's "TWINSCAN XT-1900i") under optical conditions of NA = 1.35 and dipole (σ = 0.9/0.7) through a mask pattern with 50 nm holes and a 100 nm pitch. After exposure, a post-exposure bake (PEB) was performed at 100°C for 60 seconds. Thereafter, the resist film was subjected to alkaline development using a 2.38 mass% aqueous TMAH solution as an alkaline developer, and after development, it was washed with water and further dried to form a positive resist pattern (50 nm holes, 100 nm pitch).

<Evaluation>
The resist patterns formed using the positive radiation-sensitive composition for ArF immersion exposure were evaluated for sensitivity, CDU, MEEF, number of development defects, pattern circularity, and pattern rectangularity according to the methods described below. The results are shown in Tables 5-1 and 5-2. The resist patterns were measured using a scanning electron microscope (CG-5000 manufactured by Hitachi High-Technologies Corporation).

[sensitivity]
In forming a resist pattern using the positive-tone radiation-sensitive composition for ArF immersion exposure, the optimal exposure dose was determined to be the exposure dose required to form a pattern with 50 nm holes and a 100 nm pitch, and this optimal exposure dose was used to determine the sensitivity (mJ/ ^cm2 ). Sensitivity was evaluated as "good" when it was 40 mJ/ ^cm2 or less, and "poor" when it exceeded 40 mJ/ ^cm2 .

[CDU]
A total of 1,800 resist patterns with 50 nm holes and 100 nm pitch were measured at arbitrary points from the top of the pattern using the scanning electron microscope. The dimensional variation (3σ) was calculated and used as CDU (nm). The smaller the CDU value, the smaller the variation in hole diameter over a long period, indicating better results. CDU performance was evaluated as "good" when it was 3.5 nm or less, and "poor" when it exceeded 3.5 nm.

[MEEF]
In the resist patterns resolved by irradiation with the above-mentioned optimum exposure dose, the diameters of the resist patterns formed using mask patterns with hole diameters of 52 nm, 54 nm, 56 nm, 58 nm, and 60 nm were plotted on the vertical axis against the diameters of the mask patterns on the horizontal axis, and the slope of the line was calculated and defined as MEEF. The closer the MEEF value is to 1, the better the mask reproducibility. MEEF was evaluated as "good" when it was 2 or less, and as "poor" when it exceeded 2.

[Number of development defects]
The resist film was exposed to an optimal exposure dose to form a resist pattern with 50 nm holes and a 100 nm pitch, which was used as a wafer for defect inspection. The number of defects on this wafer for defect inspection was measured using a defect inspection device (KLA-Tencor's "KLA2810"). Defects with a diameter of 5 μm or less were determined to be originating from the resist film, and the number was calculated. After development, the number of defects determined to be originating from the resist film was evaluated as "good" if the number of defects was 100 or less, and as "poor" if the number of defects was more than 100.

[Pattern circularity]
The 50 nm holes and 100 nm pitch contact holes formed by irradiating with the optimum exposure dose obtained in the sensitivity evaluation were observed in plan view using the scanning electron microscope, and their vertical and horizontal sizes were measured. If the ratio of the vertical size to the horizontal size was 0.90 or more and 1.10 or less, the hole was rated "A" (good), and if it was less than 0.90 or more than 1.10, the hole was rated "B" (poor).

[Pattern rectangularity]
The 50 nm holes and 100 nm pitch contact holes formed by irradiating with the optimum exposure dose obtained in the sensitivity evaluation were observed using the scanning electron microscope, and the cross-sectional shape of the contact hole pattern was evaluated. The rectangularity of the resist pattern was evaluated as "A" (very good) if the ratio of the length of the lower side to the length of the upper side (opening diameter) in the cross-sectional shape of the hole portion was 1 or more and 1.05 or less, "B" (good) if it was more than 1.05 and 1.10 or less, and "C" (poor) if it was more than 1.10.

As is clear from the results in Tables 5-1 and 5-2, when the radiation-sensitive compositions of the Examples were used in ArF immersion exposure, the sensitivity, CDU, MEEF, development defect performance, and pattern shape were good, whereas in the Comparative Examples, each characteristic was inferior to that of the Examples. Therefore, when the radiation-sensitive compositions of the Examples are used in ArF immersion exposure, resist patterns can be formed with high sensitivity and good roughness performance, development defect performance, and pattern shape.

[Preparation of a negative-tone radiation-sensitive composition for ArF immersion exposure, and formation and evaluation of a resist pattern using this composition]
[Example 44]
A radiation-sensitive composition (J-44) was prepared by mixing 100 parts by mass of (A-8) as the polymer (A), 12.0 parts by mass of (B-2) as the radiation-sensitive acid generator (B), 3.0 parts by mass of (B-33), 3.0 parts by mass of (cc-6) as the acid diffusion controller (C), 3.0 parts by mass (solids content) of (F-3) as the high fluorine content polymer (F), and 3,230 parts by mass of a mixed solvent of (E-1)/(E-2)/(E-3) (mass ratio 2,240/960/30) as the solvent (E), and filtering the mixture through a membrane filter having a pore size of 0.2 μm.

A composition for forming a bottom anti-reflective coating (Brewer Science's ARC66) was applied to a 12-inch silicon wafer using a spin coater (Tokyo Electron Limited's CLEAN TRACK ACT12), and then heated at 205°C for 60 seconds to form a bottom anti-reflective coating with an average thickness of 100 nm. The negative-tone radiation-sensitive composition (J-44) for ArF exposure prepared above was applied to this bottom anti-reflective coating using the spin coater, and prebaked at 100°C for 60 seconds. This was then cooled at 23°C for 30 seconds to form a resist film with an average thickness of 230 nm. Next, this resist film was exposed to light using an ArF excimer laser immersion exposure system (ASML's "TWINSCAN XT-1900i") under optical conditions of NA = 1.35 and annular (σ = 0.8/0.6) through a mask pattern with 80 nm holes and a 150 nm pitch. After exposure, a post-exposure bake (PEB) was performed at 100°C for 60 seconds. The resist film was then developed using n-butyl acetate as the organic solvent developer and dried to form a negative resist pattern (contact hole pattern with 80 nm holes and a 150 nm pitch).

The resist patterns formed using the negative-tone radiation-sensitive composition for ArF immersion exposure were evaluated for sensitivity, CDU, MEEF, and pattern circularity in the same manner as for the resist patterns formed using the positive-tone radiation-sensitive composition for ArF immersion exposure. As a result, the radiation-sensitive composition of Example 44 exhibited good sensitivity, MEEF, CDU, and pattern circularity, even when a negative-tone resist pattern was formed by ArF immersion exposure.

[Preparation of Positive-Working Radiation-Sensitive Composition for Exposure to Extreme Ultraviolet (EUV) Radiation]
[Example 45]
A radiation-sensitive composition (J-45) was prepared by mixing 100 parts by mass of (A-12) as the polymer (A), 50.0 parts by mass of (B-13) as the radiation-sensitive acid generator (B), 25.0 parts by mass of (C-9) as the acid diffusion controller (C), 5.0 parts by mass (solids content) of (F-5) as the high fluorine content polymer (F), and 6,800 parts by mass of a mixed solvent of (E-1)/(E-2) as the solvent (E), and filtering the mixture through a membrane filter having a pore size of 0.2 μm.

[Examples 46 to 93, 104 to 105 and Comparative Examples 14 to 26]
Radiation-sensitive compositions (J-46) to (J-93), (J-104) to (J-105), and (CJ-14) to (CJ-26) were prepared in the same manner as in Example 44, except that the types and amounts of each component shown in Tables 6-1 and 6-2 below were used.

<Formation of Resist Pattern Using Positive Radiation-Sensitive Composition for EUV Exposure>
A composition for forming a bottom antireflective coating (Brewer Science's ARC66) was applied to a 12-inch silicon wafer using a spin coater (Tokyo Electron Limited's CLEAN TRACK ACT12), and then heated at 205°C for 60 seconds to form a bottom antireflective coating with an average thickness of 105 nm. The positive radiation-sensitive composition for EUV exposure prepared above was applied to this bottom antireflective coating using the spin coater, and baked at 130°C for 60 seconds. This was then cooled at 23°C for 30 seconds to form a resist film with an average thickness of 65 nm. Next, this resist film was exposed to light using an EUV exposure system (ASML's NXE3300) with NA=0.33, illumination conditions: Conventional s=0.89, and a mask: imecDEFECT32FFR02. After the exposure, PEB was performed for 60 seconds at 120° C. Thereafter, the resist film was subjected to alkaline development using a 2.38% by mass aqueous solution of TMAH as an alkaline developer, and after development, the resist film was washed with water and further dried to form a positive resist pattern (30 nm contact hole pattern).

<Evaluation>
The resist patterns formed using the above-described positive-tone radiation-sensitive composition for EUV exposure were evaluated for sensitivity, CDU, and the number of development defects according to the following methods. The results are shown in Tables 7-1 and 7-2 below. The resist patterns were measured using a scanning electron microscope (CG-5000 manufactured by Hitachi High-Technologies Corporation).

[sensitivity]
In forming a resist pattern using the positive-tone radiation-sensitive composition for EUV exposure, the exposure dose required to form a 30 nm contact hole pattern was defined as the optimum exposure dose, and this optimum exposure dose was defined as the sensitivity (mJ/ ^cm2 ). Sensitivity was evaluated as "good" when it was 40 mJ/ ^cm2 or less, and "poor" when it exceeded 40 mJ/ ^cm2 .

[CDU]
A resist pattern was formed by adjusting the mask size so that a 30 nm contact hole pattern was formed by irradiating the optimal exposure dose determined in the sensitivity evaluation. The formed resist pattern was observed from above the pattern using the scanning electron microscope. The hole diameter was measured at 16 points within a 500 nm range to determine the average value, and this average value was measured at a total of 500 points at any point. The 1 sigma value was calculated from the distribution of the measured values, and this was used as the CDU performance (nm). The smaller the CDU value, the smaller the variation in hole diameter over a long period, and the better the performance. CDU performance was evaluated as "good" when it was 2.0 nm or less, and as "poor" when it exceeded 2.0 nm.

[Number of development defects]
The resist film was exposed to an optimum exposure dose to form a 30 nm contact hole pattern, which was used as a wafer for defect inspection. The number of defects on this wafer for defect inspection was measured using a defect inspection device (KLA-Tencor's "KLA2810"). Defects with a diameter of 5 μm or less were determined to be originating from the resist film, and the number of defects was calculated. After development, the number of defects determined to be originating from the resist film was evaluated as "good" if the number of defects was 50 or less, and as "poor" if the number of defects was more than 50.

As is clear from the results in Tables 7-1 and 7-2, the radiation-sensitive compositions of the Examples exhibited good sensitivity, CDU, and development defect performance when used for EUV exposure, whereas the Comparative Examples were inferior to the Examples in each of these properties.

[Preparation of a negative radiation-sensitive composition for EUV exposure, and formation and evaluation of a resist pattern using this composition]
[Example 94]
A radiation-sensitive composition (J-94) was prepared by mixing 100 parts by mass of (A-15) as the polymer (A), 20.0 parts by mass of (B-10) as the radiation-sensitive acid generator (B), 18.0 parts by mass of (D-4) as the acid diffusion controller (D), 2.0 parts by mass (solids content) of (F-5) as the high fluorine content polymer (F), and 6,110 parts by mass of a mixed solvent of (E-1)/(E-2) (mass ratio 4,280/1,830) as the solvent (E), and filtering the mixture through a membrane filter having a pore size of 0.2 μm.

A 12-inch silicon wafer was coated with a composition for forming a bottom anti-reflective coating (Brewer Science's ARC66) using a spin coater (Tokyo Electron Limited's CLEAN TRACK ACT12), which was then heated at 205°C for 60 seconds to form a bottom anti-reflective coating with an average thickness of 105 nm. The spin coater was then used to coat the bottom anti-reflective coating with the negative radiation-sensitive composition for EUV exposure (J-94) prepared above, followed by PB at 130°C for 60 seconds. This was then cooled at 23°C for 30 seconds to form a resist film with an average thickness of 55 nm. This resist film was then exposed to light using an EUV exposure system (ASML's NXE3300) with NA = 0.33, illumination conditions: Conventional s = 0.89, and a mask: imecDEFECT32FFR15. After exposure, PEB was performed at 120°C for 60 seconds. The resist film was then developed using n-butyl acetate as the organic solvent developer and dried to form a negative resist pattern (contact hole pattern with 25 nm holes and a 50 nm pitch).

The resist pattern formed using the negative-tone radiation-sensitive composition for EUV exposure was evaluated in the same manner as the resist pattern formed using the negative-tone radiation-sensitive composition for ArF immersion exposure. As a result, the radiation-sensitive composition of Example 94 exhibited good sensitivity, CDU, and pattern circularity, even when a negative-tone resist pattern was formed by EUV exposure.

The radiation-sensitive composition and pattern formation method described above enable the formation of resist patterns that have good sensitivity to exposure light and are excellent in CDU, MEEF, development defect suppression, pattern circularity, and pattern rectangularity. Therefore, these compositions can be suitably used in the fabrication processes of semiconductor devices, which are expected to become even more miniaturized in the future.

Claims (13)

an onium salt compound represented by the following formula (1);
a polymer including a structural unit having an acid-dissociable group;
A radiation-sensitive composition comprising: a solvent;

(In formula (1),
^R1 is a monovalent organic group having 4 to 40 carbon atoms.
A ⁻ is —SO ₃ ⁻ , —COO ^— or —N ⁻ —SO ₂ —R ^X. R ^X is a monovalent organic group having 1 to 20 carbon atoms.
E ¹ is —O—, —S—, —SO— or —SO ₂ —.
^R2 is a hydrogen atom or a monovalent organic group having 1 to 20 carbon atoms.
E is —O— or —NR ^Y —, and ^{R Y} is a hydrogen atom or a monovalent hydrocarbon group having 1 to 10 carbon atoms.
^R3 is a hydrogen atom or a monovalent organic group having 1 to 20 carbon atoms.
^R4 and ^R5 are each independently a monovalent organic group having 1 to 20 carbon atoms, or ^R4 and ^R5 taken together represent a ring structure having 4 to 12 carbon atoms together with the sulfur atom to which they are bonded.
^R6 is a halogen atom, a hydroxy group, a nitro group, an amino group, a carboxy group, a cyano group, or a monovalent organic group having 1 to 20 carbon atoms. When a plurality of ^R6s are present, the plurality of ^R6s may be the same or different.
m is 0 or 1. When m is 1, both R ³ -E-CO- and R ² -E ¹ - are bonded to the 6-membered ring structure to which the sulfur atom in the above formula (1) is bonded.
n is an integer from 0 to 4. 2. The radiation-sensitive composition according to claim 1, wherein in the formula (1), E and ^E1 are —O—. The radiation-sensitive composition according to claim 1, wherein m is 0. 4. The radiation-sensitive composition according to claim 1, wherein in the formula (1), R ³ -E-CO- is in an ortho position relative to R ² -E ¹ -. 4. The radiation-sensitive composition according to claim 1, wherein in the formula (1), R ² is an alkyl group, an alkoxyalkyl group, an alkoxycarbonylalkyl group, or a cycloalkoxycarbonylalkyl group. 4. The radiation-sensitive composition according to claim 1, wherein in the formula (1), R ¹ includes at least one structure selected from the group consisting of a cyclic structure, a carbonyl group, and an ether bond. The radiation-sensitive composition according to any one of claims 1 to ₃ , wherein in the above formula (1), A ^- is -SO ₃ ^{- ,} and a fluorine atom, a fluorinated hydrocarbon group, or a cyano group is bonded to the α-position carbon or β-position carbon in R ¹ relative to the sulfur atom in -SO 3 -. In the above formula (1),
The radiation-sensitive composition according to any one of claims ¹ to ₃ , wherein A ^- is ^-COO- , or A ^- is -SO ₃ ^{- ,} and a fluorine atom, a fluorinated hydrocarbon group, or a cyano group is not bonded to the carbon atom in R 1 at the α-position or the β-position relative to the sulfur atom in -SO 3 -. The radiation-sensitive composition according to any one of claims 1 to 3, wherein the content of the onium salt compound is 0.1 parts by mass or more and 100 parts by mass or less per 100 parts by mass of the polymer. 4. The radiation-sensitive composition according to claim 1, wherein the structural unit having an acid-dissociable group is represented by the following formula (3):

In formula (3), R ¹⁷ is a hydrogen atom, a fluorine atom, a methyl group, or a trifluoromethyl group.
R ¹⁸ is a monovalent substituted or unsubstituted hydrocarbon group having 1 to 20 carbon atoms.
R ¹⁹ and R ²⁰ each independently represent a monovalent substituted or unsubstituted chain hydrocarbon group having 1 to 10 carbon atoms, a monovalent substituted or unsubstituted alicyclic hydrocarbon group having 3 to 20 carbon atoms, or a divalent alicyclic group having 3 to 20 carbon atoms formed by combining these groups together with the carbon atoms to which they are bonded.
L ¹¹ represents ^* -COO-, ^* -L ^11a COO- or ^* -COOL ^11a COO-, wherein ^{L 11a} is a substituted or unsubstituted alkanediyl group or arenediyl group.
* indicates a bond to the carbon atom to which R ¹⁷ is bonded.) a step of directly or indirectly applying the radiation-sensitive composition according to any one of claims 1 to 3 to a substrate to form a resist film;
exposing the resist film to light;
and developing the exposed resist film with a developer. The pattern formation method according to claim 11, wherein the exposure is performed using an ArF excimer laser or extreme ultraviolet light. An onium salt compound represented by the following formula (1a):

(In formula (1a),
R ^1a is a monovalent organic group having 5 to 40 carbon atoms.
A ⁻ is —SO ₃ ⁻ , —COO ^— or —N ⁻ —SO ₂ —R ^X. R ^X is a monovalent organic group having 1 to 20 carbon atoms.
E ¹ is —O—, —S—, —SO— or —SO ₂ —.
^R2 is a hydrogen atom or a monovalent organic group having 1 to 20 carbon atoms.
E is —O— or —NR ^Y —, and ^{R Y} is a hydrogen atom or a monovalent hydrocarbon group having 1 to 10 carbon atoms.
^R3 is a hydrogen atom or a monovalent organic group having 1 to 20 carbon atoms.
^R4 and ^R5 are each independently a monovalent organic group having 1 to 20 carbon atoms, or ^R4 and ^R5 are combined with each other to form a ring structure having 4 to 12 carbon atoms together with the sulfur atom to which they are bonded, provided that in the ring structure, the ring containing the sulfur atom in formula (1a) is fused with two other rings to form a tricyclic structure, and when the ring containing the sulfur atom in formula (1a) contains a heteroatom other than the sulfur atom, the heteroatom is an oxygen atom or a nitrogen atom.
^R6 is a halogen atom, a hydroxy group, a nitro group, an amino group, a carboxy group, a cyano group, or a monovalent organic group having 1 to 20 carbon atoms. When a plurality of ^R6s are present, the plurality of ^R6s may be the same or different.
m is 0 or 1. When m is 1, both R ³ -E-CO- and R ² -E ¹ - are bonded to the 6-membered ring structure to which the sulfur atom in the above formula (1a) is bonded.
n is an integer from 0 to 4.